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Repairing Gelcoat

by Steve Kennerly

Here is a step by step for repairing scratches in gelcoat. This would apply to deeper and/or wider scratches, where the raw fiberglass is showing through. However, you can use this same method on small scratches that are too deep to buff out, but do not go all the way to the raw fiberglass. The only difference would be that less agressive sanding is required, during the initional preparation and the process of sucessively overfilling with several coats of gelcoat will take fewer over coatings (perhaps only one coat will be required). With a little practice, you should get very professional looking results, using these steps.

Here's what you'll need to do this repair.

  • gelcoat or gelcoat paste (I prefer plain old gelcoat, because I have found that it doesn't seem to yellow with time as much as the paste.) If the scratch is really deep, or wide get some of each
  • mek hardener
  • small can of acetone
  • plastic cup, or mixing board (board is also plastic)
  • squeegee (the plastic kind for spreading bondo) or a tongue depresser, or a small plastic putty knife will work, but not as easily.
  • roll of waxed paper
  • sand paper (grits below)
  • hard rubber sanding block
  • spounge or spounge rubber sanding block
  • pair of chemical resistant, rubber gloves
  • roll of masking tape and some paper (news paper or masking paper)
  • a few clean rags

For sandpaper, you will need a few different grits:

  • #180 to #220
  • (dry production type, aluminum oxide works best, which is a very light grey or whitish on the gritty side. the brown stuff doesn't cut the gelcoat too well)
  • #400
  • #600
  • #800 to #1200
  • #1500 to #2000
All of the above, starting with the #400 are wet or dry type.

First, clean the area well. If it is at or near the waterline, particularly. Be sure there is no oily residue around, or in the scratch. Now check the damaged area for loose chips of gelcoat along it's edges. I usually do this by running a fingernail along the edge. You will be able to feel loose chips and break them off. Go easy..... a sliver of gelcoat jabbed up under your fingernail doesn't feel very good. ;)

Clean the area very well again with acetone and a clean rag. Turn or fold the rag a number of times and wipe the area with the clean side, wetted with acetone. This removes any residual dirt, oil and wax. Clean right down in the scratch and around it so there is a perfectly clean area of about 2 or three inches all around it. WEAR THE RUBBER GLOVES while working with the acetone. It soaks into skin readily and is very hard on your liver.

Now, using the dry sanding paper, sand the scratch well. It helps on scratches to wrap a little paper around a pencil, pen or other small, round object. You are sanding just in the scratch itself and along it's edges here. Don't sand an area around the scratch. Just sand in the scratch itself and smooth any jagged edges. Once again check for loose chips of gelcoat along the edges, as you sand. Try to give the scratch a consistant shape and depth along it's length in this process. The best shape would be a V, however a cupped shape will work also. Don't deepen or widen the scratch a lot trying to get this shape however. The gelcoat is quite forgiving.

Using another clean rag, wash out the scratch with acetone again. (remember the gloves)

Now, (finally) you are ready to start filling the scratch with new gelcoat. Remember, in gelcoat repair, as in the auto paint and body industry, the repair will only be as good as the preparation, so follow the above steps carefully.

If the scratch is small, you may be able, at this point to do one filling of gelcoat paste and be done with it. If it is deep and/or very wide a few fillings may be required. If you are using plain gelcoat, you will surely have to coat the scratch several times, unless it is very small. If it is a very large and deep scratch you will start filling with the paste and use gelcoat for the final coat or two, for a very nice repair.

Take your plastic squeegee and trim off a piece about a inch wide (more or less), so you have a little narrow one. Keep the, probably wider part of the squeege handi though.

Mask off an area around the scratch, particularly below it, so any drips of gelcoat will not end up on the good finish. Don't mask too close to the repair area itself though.

Mix a small amount of paste, or gelcoat in the cup or on the mixing board. follow the directions carefully. I find in warmer climates, a little less MEK hardener will keep the gelcoat from kicking too soon. Apply the gelcoat, or paste with the small squeegee. You are not trying to completely fill the scratch here. You will repeat this process several times, until the scratch is filled completely. Let each coat cure to a tacky feel, before you add another coat.This way you won't have a blob of gelcoat come dripping out of the scratch. You will then add a final coat that brings the surface of the repair, just higher than the original finish. We're not trying to make the repair look perfect here. We are just filling in the scratch. When you feel the repair is equal to or slightly above the original surface, cover the final coat of gelcoat with a piece of waxed paper, while it is still wet. Smooth out the paper with the leftover clean squeegee and tape it in place. Let things stand as they are for the next 20 minutes, at least. If you can't get back to it, at this point, until the next day, that's okay too.

When cured, remove the waxed paper and inspect the repair. There will be some shrinkage of the gelcoat and it may still be low in places. don't try to fill these low spots yet.

Take the #400 paper and cut a piece to fit the hard rubber sanding block. You can have a bucket of water handi to dip the block in as you go, or you can run a trickle of water from a hose over the area as you sand. You must use the sanding block in these following steps, to make the repair look right. Don't sand with your hands and paper alone. The end result will look wavey. You will start to be able to see where the paper is sanding and where it isn't, by drying the area off every now and then and looking closely. The areas being sanded will be dull and the low spots and original finish around the repair will still be shiney. If you start dulling the original finish around the repair, at this point, stop sanding and add another coat of gelcoat. When the area is reasonably fair, but has a few low spots (you will be able to see them easily now, as they will be little shiney places in the middle of your sanding field) add another coat of gelcoat. Sand lightly again with the #400. Keep the water trickling, or keep dipping the block in the bucket as you go. This step is not the final one, so don't over sand. The original gelcoat is fairly thick, but it does have it's limits.

Now, rinse off the repair well, with clean water (not the water in the bucket). Remove your #400 grit from the block and rinse the block well too. Install the #600 paper and wet sand the repair with it for awhile. The idea here is to remove the scratches created by the #400 paper. If there yet remain some low spots, this will be the final chance to fix them. They should be indeed small, or non-existent by this time. You will also notice that the #600 paper will cause the repair to start taking on a dull shine again.

If, at this point you are happy with the look of the repair (here we're looking for a flat, smooth look that blends out into the original surface), you can go to the next step. If the repair is still lumpy, and or uneven, go back a couple steps. You'll have to scuff the area with the #180 or #220 some if you need to add some more gelcoat. Add a coat and cover with waxed paper again. Follow steps to this point and continue.

You are now ready to burnish the area with the #800 to #1200 grit paper. You are still wet sanding and you are still using the sanding block. You are only trying to remove scratches caused by the last grit used, so this won't take long at this point. You will see the glow, or shine getting glossyer when you dry off the repair. BTW Stop and dry the area regularly throughout the finish sanding process to check your work.

After using the #1200 grit, some will go ahead and buff and wax the repair. It will actually be easier to go to the next grit and clean up the repair a little more. But you have to look at the overall finish. If the rest of the boat looks like glass, take the next step. If not stop here and get the whole boat buffed out.

If you're moving on, go to the #1500 to #2000 paper. Here you will, perhaps find that the hard rubber block seems to stick, or drag on the surface. If so, wrap the sandpaper around the spounge instead and wet sand this way.

You should now have a repair that nobody will ever know about, except you and me ;). The only thing will be the color match. This, I usually don't worry about too much. If it is a reasonable match, you can't really see it from more than a foot away and gelcoat tends to yellow a little with time. If the repair looks whiter than the rest of the boat give it a season. It'll probably match itself in nicely. Polishing and waxing the area is, of course, the final step. The wax itself will even yellow the new gelcoat slightly.

Gelcoat is very forgiving stuff, so don't worry if it doesn't come out quite right the first time. At worst, you will have a new gelcoat layer where there was raw fiberglass and you can always go back to this area, touch it up and practice the process over again, until you get it just the way you want it to look. In the mean time, you are learning a skill that boat owners pay dearly for, everyday. You will get a feel of how this best works for you and you'll never worry about the odd nick your boat receives, 'cause you know it can be fixed easily. The process does sound very involved on paper. This is partially because I've tried to break it down into detailed steps and partly due to my long windedness ;). However, it is really easy, once you get the hang of how to do it. Good luck on this project and if you get stuck at some point, or things don't go as planned, do feel free to drop me a line, with questions. Hope this has been helpful.

"Floatin' my boat floats my boat"

Best Breezes to You and Yours,
Steve K.

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