Gel Coat Repair

George Van Parys

Mar 1, 2000
RO Number
From a major boat manufacturers former head of the Gelcoat dept. (Friend of mine)

The first thing to do is make sure all of the mud work is straight. The easiest way to do
this is with an air file. If an air file isn't available then any kind of long block will work.
They are inexpensive, and they work well. Block all of the mud work until it feels smooth
when you run your hand over it. Then take some black spray paint (any kind of cheap
paint will work) and spray over your body work. Just cover the mud, you don't need to
put a heavy coat on. When this dries, Go over it with the long block with 80 grit. This
will show you the high and low spots in your mud work. Fill any low spots with more
putty and block that down. Spray it again with the paint and when all of the paint sands
off in a uniform manner than the mud work should be straight enough. When you are
blocking your body work avoid the temptation to put uneven pressure on the long block in
order to sand a high spot down a will show in the finished product.

Once you are satisfied that it's straight enough blow the entire area off with a high
pressure blower. This will open any pits up that have filled with dust. If the pits are large,
say bigger than a pin head, then you need to fill them with putty and block them down.
Look at any areas surrounding the body work. If you see bare glass, then it most likely
will have very tiny pin holes in it. Take a little gelcoat and mix it with cabosil until you
have a smooth paste about the consistency of cake frosting. Catalyze it and with your
finger smear some on any bare glass you see, and any small pits in the mud work. Allow
this to dry and sand it smooth with 220. You may still have some small pits here and
there, but if they are small it shouldn't be a problem. Next you want to take 220 and sand
the entire area extending about 4 to 6 inches into the area of good gelcoat. It looks like at
least 2 colors will be sprayed so take care to tack the white area, if you don't tack it you
may end up with a pink halo around the white spray. Mask off the area you want to spray
first. If you are shooting white first, mask off the red area, and vice versa when you spray
the red. If there is any hardware in the area to be sprayed, it's best to remove it if at all
possible, making sure to get any sealant off also. Paper the area up very
good.........overspray has a way of finding the tiniest hole in the masking paper. In fact it's
best to cover the whole boat before you spray, and keep it covered until the whole job is

One the covering and masking is finished you can shoot the gelcoat. There are quite a few
additives to mix with the gelcoat to thin it and aid in the drying process, but if none are
available acetone works pretty well. When you thin gelcoat with acetone start with just a
few drops, and add it like that until you get the consistency of a thin milkshake. When
satisfied with the consistency catalyze it with MEKP. the proper amount is about a 2%
mixture. 5cc's to an 8 oz cup of gelcoat will give you about 8 to 10 minutes of spraying
time. When you shoot the gel, apply it in long even coats. If pits appear, avoid holding
the gun in one spot to fill them, just keep moving back and forth. When you empty the
gun, or it starts to kick, clean it immediately with acetone, dry it and mix another batch of
paint. If you still see pits, spray a spot on you masking paper and using a razor blade fill
the pits with the gel you sprayed on the paper. Then once again spray the area in long
smooth even coats. Allow the white area to tack up, remove the paper from the red area
and spray it the same way. One word of caution, when you get to the edge of your repair
area, do not spray on the tape line of the masking paper. Spray up to it, but not on don't want any sort of visible paint line.

Give your repair at least a 24 hour period to thoroughly dry, possible 48 hours depending
on the temperature. Without the proper additives, gelcoat take a very long time to cure,
so it's best to wait an extra day if necessary. When you are sure it's fully cured remove
the masking paper from the repair area and wipe the whole repair with a rag soaked with
acetone. This will remove any contaminates that have come to the surface. Once again,
make absolutely sure it's dry, the acetone will remove the gelcoat if it isn't dry. Even
when it's dry you will see some pigment on the rag, but that isn't a problem if the paint is

Now you must work the spray down to that nice hard finish. Start with 320 sandpaper on
a block or air file and work the spray the same way as the mud work. The idea here is to
smooth the gel and remove all traces of orange peel. This is tedious work and you will
use a lot of sand paper, but for a good straight finish it's necessary. After you have
removed the orange peel switch to 600 sandpaper. the idea here is to remove the 320
sand marks, and this won't take quite so long. When you think you have sanded enough
buff the repair with a rather heavy compound, some thing like 720. this will take out a lot
of sand marks and put a fair shine on the finish. take a trouble light and look closely at the
repair. Look for orange peel, paint lines and pits, if you find any sand them out. After
you have all of you sand marks out, put on a new buffing pad and buff the repair with a
fine finishing compound. 2000 grit is excellent.

If you happen to sand through in places these can be resprayed using the same processes
as before, only on a smaller scale. Look the repair over and make sure you are satisfied
with it. Do not expect it to be a perfect repair. Gelcoat is extremely hard to repair with
no traces. A slight waviness is to be expected, so don't expect results that can only come
from years of experience. Remember how it looked before you started and be proud of
the job the accomplished.

A word of caution when dealing with polyesters, Mekp, and acetone. Polyesters WILL
COMBUST. Any unused gelcoat that has been catalyzed should be poured into a bucket
or can that is filled with water. The same goes for putty, even if it's hard, put it in the
water, and let it sit for several hours before you throw it away. MEKP is very reactive
and should be kept away from all metals. Acetone is extremely flammable and should be
treated as such. Any rags with acetone in them should be spread out and allowed to air
dry, don't just throw them away of toss them in a pile. It would be a shame to put so
much work into this repair only to have the boat damaged further in an accidental fire.