Teach me about seam grinding puh-lease

[Alex]

My intention is to run in the gelcoat for a while before paint - from the pictures I've seen, it is of excellent quality (versus others like FFR). Therefore, I'd like to keep 'damages' down to a minimum..........you don't want to see what my cobra body looked like when I started cutting out stuff like roll-bar holes and enlarging the side exhaust :drunk: :laugh:

So right now I've paid Fran to make my body near paint ready, with pretty much just the seam work left. Now, I've never done any type of work before on gelcoat, so I have no idea what I'm facing. If anybody could give me a rough list of steps, I'd be grateful - so like if you were sitting there staring at your SLC's seams right now, what steps would you do to knock them down and then polish them out so the gelcoat looks perfect. (e.g., if this were wetsanding a fresh paint job, you'd wetsand by hand (atleast I do) with 1500, then 2500, then use a d/a like a flex 3401 (or rotary, although I almost never use one) and a wool pad and cutting compoudn (e.g., Menzerna SIP), followed by a white pad and a light polishing compound (e.g., Menzerna 106).

Do I want to use an orbital sander up until some point? What grade paper? Wetsand? etc... etc.... (basically starting with a lot of detailing knowledge, but 0 gelcoat knowledge)

 

[Allen English]

My intention is to run in the gelcoat for a while before paint - from the pictures I've seen, it is of excellent quality (versus others like FFR). Therefore, I'd like to keep 'damages' down to a minimum..........you don't want to see what my cobra body looked like when I started cutting out stuff like roll-bar holes and enlarging the side exhaust :drunk: :laugh:

So right now I've paid Fran to make my body near paint ready, with pretty much just the seam work left. Now, I've never done any type of work before on gelcoat, so I have no idea what I'm facing. If anybody could give me a rough list of steps, I'd be grateful - so like if you were sitting there staring at your SLC's seams right now, what steps would you do to knock them down and then polish them out so the gelcoat looks perfect. (e.g., if this were wetsanding a fresh paint job, you'd wetsand by hand (atleast I do) with 1500, then 2500, then use a d/a like a flex 3401 (or rotary, although I almost never use one) and a wool pad and cutting compoudn (e.g., Menzerna SIP), followed by a white pad and a light polishing compound (e.g., Menzerna 106).

Do I want to use an orbital sander up until some point? What grade paper? Wetsand? etc... etc.... (basically starting with a lot of detailing knowledge, but 0 gelcoat knowledge)

Alex, I did all of the seam and body work to my FFR Roadster and it didn't save me anything. I think its best to leave the body work to the painter in my opinion. If you feel ambitious why don't you fix the seems and block out the body and get it in primer state then have the body shop finish it when you are ready?

 

[Allen English]

Call me if you want to talk body work (503-568-8344) Been there and done that.

 

[Alex]

If you feel ambitious why don't you fix the seems and block out the body and get it in primer state then have the body shop finish it when you are ready?

Once the seams are knocked down it should be easy to polish out the gelcoat so it looks driveable (white gelcoat) until I'm ready to go to paint - it's just that I don't know how to get to that point.

Quality fiberglass work is hard to find around here, so it comes at a steep premium. Worth it, but for most of the pre-paint prep I think can handle myself (since it shouldn't be like my FFR, where pieces were warped and didn't look like they even belonged on my car)

 

[Cliff Beer]

Alex, if you do the seam work yourself then you're going to end up with a fair amount of visible work in the adjoining areas (to the seam). In other words, inevitably there will be a good amount of fairing in to do which will impact the gelcoated areas, either will building up, or some amount of sanding into, the gelcoat. I think it will be very difficult, if not impossible, to preserve the surrounding gelcoated areas and still get the seams properly set.

The seams may not be that bad - Fran's team does good work. You could probably run in the gelcoat for a bit as-is and then take the car to the painter (or do it yourself) for seam work and paint at some point down the road when you're ready.

 

[Alex]

Alex, if you do the seam work yourself then you're going to end up with a fair amount of visible work in the adjoining areas (to the seam). In other words, inevitably there will be a good amount of fairing in to do which will impact the gelcoated areas, either will building up, or some amount of sanding into, the gelcoat. I think it will be very difficult, if not impossible, to preserve the surrounding gelcoated areas and still get the seams properly set.

Soooooo (and this may be a stupid question), but can't you mask off the surrounding areas of the seam, use an orbital to knock down the majority of the seam, then a sanding block to hand sand down the seam flush.

Then wetsand the area and surrounding area with 2000gritt, then a heavy cutting compound and a wool pad to remove the sanding tracers?

 

[Dave]

Soooooo (and this may be a stupid question), but can't you mask off the surrounding areas of the seam, use an orbital to knock down the majority of the seam, then a sanding block to hand sand down the seam flush.

Then wetsand the area and surrounding area with 2000gritt, then a heavy cutting compound and a wool pad to remove the sanding tracers?

Alex,
I am also planning on just polishing out the gel-coat at first and the steps you listed are exactly what I was planning on doing too. If there is a better way, I'd love to know it. I'm not looking for perfection, just decent finish at first until the car does get painted.

 

[Gregg]

Soooooo (and this may be a stupid question), but can't you mask off the surrounding areas of the seam, use an orbital to knock down the majority of the seam, then a sanding block to hand sand down the seam flush.

Then wetsand the area and surrounding area with 2000gritt, then a heavy cutting compound and a wool pad to remove the sanding tracers?

I would lose the orbital and do it all by hand. Use a sanding block and cut the paper into a strip, probably about one inch wide, affix that in the center of the block and sand the seam. It will lessen the damage to the surrounding gelcoat but should still have the benefit of the full block for straightness. If need be, cut the paper in the shape of a capital "I" to affix it to the sanding block. You can probably cut the paper into a strip and affix it along one edge with the same results. Have fun.

 

[Jac Mac]

Speak to your painter before you try to apply any polish to bare f/glass, polish will get into pin holes in the resin etc & possibly play havoc with any attempts to paint it at a later date...then you will be jumpin up & down calling the paint guy a **&^%$ when it was really the clever fella with the tin of polish.

 

[Alex]

I would lose the orbital and do it all by hand. Use a sanding block and cut the paper into a strip, probably about one inch wide, affix that in the center of the block and sand the seam. It will lessen the damage to the surrounding gelcoat but should still have the benefit of the full block for straightness. If need be, cut the paper in the shape of a capital "I" to affix it to the sanding block. You can probably cut the paper into a strip and affix it along one edge with the same results. Have fun.

Good ideas - for sanding block, this is what you mean, right?

Also, any ideas what grits to use? For example, do you use 1500 for allt he work, or do you start with something really aggressive like 100 when you start knocking down the seams, and switch to 1500 once you start getting closer to flat?

Speak to your painter before you try to apply any polish to bare f/glass, polish will get into pin holes in the resin etc & possibly play havoc with any attempts to paint it at a later date...then you will be jumpin up & down calling the paint guy a **&^%$ when it was really the clever fella with the tin of polish.

Hmmm, you do make a good point - however, before applying any primer/etc..., the painter should use some form of wax-removal wipedown (e.g., isopropenal alochol bath) to remove any polishing oils/traces/etc... left behind, no?

 

[Cliff Beer]

Well, I don't know guys, I've did a lot of body work when I was younger and when it comes to seams, to do a good job you have to both a) make the gap consistent in width, and b) level the relative (adjoining) heights once the panels are final fitted. Doing b) properly requires feathering in the heights into the surrounding areas, and that's means extending the sanding and filling some several inches (or more) back from the seam. I guess I don't know any other way to do it right other than that.

 

[Jack]

On my SL-C all of the mold seams are gelcoat, proud of the body with no gaps to fill. 80 grit on a block to start, then work the grits to about 600. A coarse rubbing compound, followed by a finer compound will bring the gelcoat up to an acceptable shine. Lot's of work! Stay away from rubbing compounds containing any waxes and you'll be okay. Before painting the car, de-wax anyway and wetsand to 400 before primer.

 

[Eddie]

Alex,
The old fashioned way was to use a SAFTEY razor-blade...wrap electrical tape equally on both ends to the height off your seam, the exposed centre of the blade now removes the seam..remove one turn of the tape & repete & so on untill you are at a height on your bodywork that you can then sand... never sand in a straight line but at an angle of 45 deg. across your seam. Hope this helps.
Eddie.

 

[EGLITOM]

Hi Alex,

check Eastwoods they have excellent quality blocks and sanding paper ( RHINO).

Other than that i would do it by hand and very carefully. Non wax polishing compound is the way to go. THe SLC´s gelcoat qualitiy is excellent and if you are patient enough it will pay out.

Another idea ( if you don´t want to spend the time on polishing)is to cover the grinded areas with some graphis applied via foil.

TOM

 

[Alex]

Other than that i would do it by hand and very carefully. Non wax polishing compound is the way to go.

I don't think there are any heavy cut compounds that will remove 1500-2000 gritt that contain waxes/silicones though (unless they're just filling in and not actually levelling)?

What I've had excellent success with, both on clearcoat and gelcoat, for 1500 sanding scratches has been menzerna SIP

Menzerna Super Intensive Polish, Menzerna PO 83 , Mernzerna PO85RD 3.02

 

[Tom]

Do all sanding by hand. I used a hand sander as pictured above and got the heavy ridge knocked down with 320 grit wet and a very light touch. Then I eventually worked down to 2000 grit.

The main thing I discovered is that once you are done with a particular grit, you have to towel dry the piece to get a good look at the scratches. If it stays wet, you don't see them until you've moved onto a finer grit which will have difficulty taking them out.

It also helps to have some spare gelcoat to do whatever repairs are needed at the moment. I typically would grind out the offending area a mm or two, touch some styrene to the ground out area to prime the spot, then lay down fresh gelcoat. Which of course would be followed by wet sanding all over again. Then machine polishing (which I did not handle).

It's not show quality, but it came out pretty nice, and it will look just fine on the track.

 

[Alex]

Do all sanding by hand. I used a hand sander as pictured above and got the heavy ridge knocked down with 320 grit wet and a very light touch. Then I eventually worked down to 2000 grit.

It also helps to have some spare gelcoat to do whatever repairs are needed at the moment. I typically would grind out the offending area a mm or two, touch some styrene to the ground out area to prime the spot, then lay down fresh gelcoat. Which of course would be followed by wet sanding all over again. Then machine polishing (which I did not handle).

2 questions

1 - What grits did you use. I.e., 320 ... ? ..... 2000 (I have no idea how to jump the numbers from 320, I mostly use 1500/2500 on wetsanding clear)

2 - For gelcoat repair, what products did you use? You mention sytrene (no idea what that is.....yet) but are there any particuar brands to buy that are really good, and others that are crap? To learn gelcoat repair, would it be feasible to take a piece of fiberglass I have (e.g., old fiberglass hood) and coat it in gelcoat for practice? Kinda like to practice painting you'd take an old hood, sand it down, then primer and spray and examine your work.

Machine polishing is easy as pie for me (I've spent years developing the art), it's trying to fill in the blanks before I get to that step, haha.

 

[Tom]

i wasn't too slavish regarding the step up in grit. Looking back, I used some 220 on spots that I really want to grind back in preparation for laying down new gelcoat. For just sanding out seams, I started with 320 on the sanding block. Then I stepped up to 600, 1200 and 2000. Once the seam was flattened, if the surface was curved (as most are on a T70) I would hand sand without a block and a light touch.

GD sent along spare gelcoat, so I had the appropriate supply of that. The gelcoat keeps, but the catalyst does not, so that needs to be replaced if over a few months old.

Styrene is the solvent used in polyester gelcoat, and dabbing the old finish before laying down gelcoat helps ensure that it will properly bond to the old gelcoat. You can get all the materials at US Composites.

I didn't do big swaths of gelcoat (which could be sprayed I'm sure), I was only doing some odd edges, shipping damage etc. You can definitely practice on a piece of fiberglass. One thing you need to take account of is that the surface of polyester will not cure in the presence of air. If you want to lay gelcoat on top of something, you also have to mix wax mold release into the gelcoat. Cure is also affected by heat and humidity.

 

[Alex]

So let's say you had a piece of fiberglass (e.g., part of a factory five cobra body) and wanted to try tou skill with gelcoat. Would a rough lists of steps be (feel free to add/correct anything)

1)Clean the area with some solvent (would something like isopropenol alcohol be the right thing to use - it strips a lot of wax/oils/etc....)

2) Wipe the desired area down with Styrene

3) Mix gelcoat with a catalyst (?What are the more popular catalysts to use, and what proportion is usually recommended?) and then ??mix that mixture with some wax mold release?? [?What proportion?)

4) Let it cure (?Does it cure faster in warmer or colder temperatures?)

5) Once hard (?how do you know when it's totally done drying?), sand and polish

Thanks

 

[Tom]

Roughly right, though you would want to put some tooth in the gelcoat to be covered - say 200 or 320 grit paper rough.

I use acetone to clean off parts. It also works to take the sticky residue off cured gelcoat that didn't have enough surfacing wax to cure completely in air.

mix your gelcoat with methyl ethyl ketone peroxide catalyst (otherwise known as MEKP) and add the surfacing wax liquid as per recipe provided. US Composites provided the recipe, if I remember correctly it's 20 drops of MEKP per ounce gelcoat, and the surfacing wax is 10 drops per ounce gelcoat. You definitely would want to confirm those proportions though.

A quick wipe with styrene (which is pretty aromatic) and lay down the gelcoat. Warm weather speeds up the cure. When cool (70 degrees as opposed to 90 degrees), I also found it didn't hurt to add a little more MEKP and surfacing wax than the recipe (+10-20%?) because the surface can't trap the heat, which slows the reaction down. High outside temperature seems to affect pot life more than what you actually want to coat.

I leave the part alone for a few hours and it should be cured and dry to touch. If the surface remains sticky for a long time, you can also try covering with saran wrap to get the surface cured. And if that doesn't work, wipe it down with acetone to get rid of the stickiness.

Once completely cured, you can start sanding.

The guys at US Composites can also help you with tips etc.

Give it a shot on some scrap fiberglass.