July 1999 Telltale page 6 - by Tom Winlow and Marcel Laroche.
The thought causes brave people to run for the hills.
It need not be so intimidating if you have a bit of time and some patience when it comes to the color matching steps. Doing these repairs can be quite rewarding. The worst that can happen is that you have to have someone like Scott or Warren come along later and help you fix a bad color match.
The first thing to do is to buy a dremel tool with one quarter and half inch sanding drums. Wearing a dust mask and eye protection, grind the gelcoat away along the crack. You will be making a smooth round depression about one quarter to three eighths wide and as deep as the gelcoat. The bottom of the groove will be the underlying glass laminate.
Don't cut into this laminate. If you see the crack in the laminate call an expert for a consultation. Make sure that you get to the end of the crack. If not, it will have a place to start again. Check this by wetting a clean rag with Acetone and wiping the newly cut grove and then looking closely at the clean ends to see if the crack extends into the remaining gelcoat. Keep grinding till the crack no longer exists.
So now instead of an almost invisible crack you have this huge groove. The next step is to fill up the groove. I like to tape around the groove leaving about 1/8 around the edge. This limits the sanding to be done later. Clean the groove with Acetone or Styrene. Then mix up some Marglass. This can be obtained along with other supplies from any automotive supply shop. Use a cream hardener. Using a plastic spatula (flexible) push the mix into the groove. Do not overfill as when it hardens you need to sand it so that it is slightly lower than the surrounding surface. Start with 120 grit paper for this and buy the best you can get, such as 3M or the Lee Valley papers. As there is a lot of sanding to do, buy some nitrile gloves from Lee Valley to protect your fingertips.
Now you have a smooth green line where there was only a crack, but keep the faith it will get better. The next step is to cover the Marglass with a finishing putty such as Bodylite or Bondo. This step provides the perfectly smooth surface on which to spray the gelcoat later. Proceed with the Bodylite as before using a flexible spatula, let it harden, then sand. You may need to do this a couple of times to get the surface perfect. At the same time you will be moving to 320 grit wet paper making sure no pin holes remain and the surrounding edges of the old surface are sanded a bit as well. At this stage the groove will be a kind of pale red color and will be absolutely smooth with no bumps or hollows when you run your fingers along or across it. Now for the gelcoat.
But some white gelcoat and some color additives, typically red, yellow, blue and perhaps a brown. Take half a pint or so of the gelcoat in a glass jar and by mixing in very small amounts (a drop on the end of a toothpick is a useful amount) of the colors tint the white Gel to the color of the boat. Sand an area near the repair with 600 grit wet paper to give yourself a clean reference. When you think your close, take a bit of the colored gelcoat on your finger a smear it on the sanded reference area. You will see whether you're close or not. Do this slowly and keep testing. When you think you have it, take a tablespoon of the colored gelcoat, catalyze it and smear a bit on one side of the sanded reference area. When it hardens, sand it and this will show immediately whether it is right. My experience says that the tendency is to make the mix a little lighter than the older surface so keep that in mind. When you are happy, it is time to spray or brush the new gelcoat on the repair. You can buy small sprayers using cans of compressed air as propellant. You will need to thin the Gel with Styrene to allow the sprayer to work. Tape the area leaving about half an inch around the repair.
Cover the surrounding area with newspaper to protect from overspray. Don't forget to add the wax to the mix. Alternatively you can buy a product called PVA which is essentially mold release wax and this can be sprayed over wet gelcoat to enable a proper cure. Once it has cured, all that remains is to carefully sand the gelcoat repair with 400 then 600 wet paper, polish with a polishing compound and finally give it a coat of wax.
It sounds like a lot of work and it is, but the finished product is worth it.