As Spring time rolls around and temperatures begin to rise, the thought of repairing that eyesore that has been hiding under shrink wrap may come to mind. Almost everyone I know at one time or another has forgot to set out fenders when mooring and consequently scratched (or worse!) the side of their boat. Nothing serious enough to compromise hull integrity, but an eyesore none the less. There are a few options you can take for repair. It all depends on how much time and effort you are willing to exert. Although the more time you spend, the higher the quality of results.
For all repairs the first and most important step is proper surface preparation. Lightly sand repair area with coarse sand paper, if repair has loose chips they must be removed. If there are scratches and cracks from the impact, use a Dremel tool to open them up. I know it may sound a little crazy to make a damaged area even larger, but if you simply cover the cracks with gelcoat they will reappear through gelcoat in a matter of days. Worse yet they may continue to crack and you'll end up with a 12 inch crack when the initial crack was only 2 inches long. Once sanded wipe with acetone to remove any dust or wax. Use care to not get acetone on rub rails or anything other than repair, it will stain and discolor anything that is not fiberglass.
The first option is to simply fill the scratch or chip with a cabosil/gelcoat putty. The putty mixture will fill the repair quickly with just one application, but the finished result will have numerous tiny pin-head sized holes. These holes will not let water intrude but will make the repair much more noticeable. To make putty mixture simply mix cabosil and gelcoat in a paint cup to peanut butter consistency. ALWAYS wear a respirator or dust mask when handling cabosil, even in a well ventilated area - it can be extremely harmful to your lungs. Once putty is mixed, apply with a plastic squeegee or spreader until level. Once cured lightly sand and buff.
Another option is to paint gelcoat on to repair with a series of at least 2 coats with a foam brush. Bristle brushes tend to leave brush strokes and do not have the tapered tip that allows a more exact application. Depending on air temperatures the second coat may be applied in as little as an hour after initial coat. Once the repair is sufficiently filled and cured, sand with 400 grit sand paper and a sanding handblock. Once repair is level sand with progressively finer grits of Wet/Dry sand paper and buff for a glossy finish.
The final and most labor intensive method of repair is to apply gelcoat with a Preval Spray Gun. This is the preferred method of professionals. It is quite labor intensive, but the end result will make it hard to detect any signs of a repair. Now is the important time of area preparation. Over-spray can quickly make a small repair into a giant mess. Use a drop cloth and masking paper in the general area of the repair. First mix up a cabosil and gelcoat putty mixture to consistency of peanut butter and apply with a plastic squeegee or spreader until level. Once cured lightly sand with medium grit sand paper and wipe with acetone. Its now time to put on your respirator with vapor cartridge and safety glasses.
Mix gelcoat and MEKP together in a separate paint cup first then add a fair amount of acetone and stir all three VERY well in the separate cup. [Gelcoat suppliers recommend thinning gelcoat with styrene rather than acetone for best results] Then use a paint strainer and pour the mixture into the preval cup. It has to be fairly thin to pass through the strainer or you will end up with a big mess so add a fair amount of acetone. The pick-up tube of the sprayer has a very fine screen that can easily get clogged, so mixture must be pretty thin. I have heard of some people removing the screen, but I would not recommend it. Test spray a small amount onto the masking paper to check the flow and prep the spray unit. Apply a light tack coat wait a few minutes. Keep swirling the bottle around continuously between coats or acetone and gelcoat will separate. Then apply a second heavier coat, but try to not to go too heavy around edges and over taped areas. Keeping it light toward the edges will reduce any transition line when you remove the tape. After a few minutes spray a third and final coat.
When you are done spraying, clean the cup with acetone and a rag. Once the cup is clean pour a little clean acetone in the cup and spray until clear. If you do this you will be able to use the unit for multiple jobs, but I would not suggest using a different color. I recommend giving the repair a full 24 hours to cure and harden. Once cured lightly wipe repair with acetone and a rag to remove surface wax. If you do not do this, the wax will gum up on your sand paper and be very difficult to sand. Start with 600 grit sand paper with a sanding handblock and sand lightly. Once smooth wet sand with 800 or 1000 grit. Use care not to over sand or burn through sprayed gelcoat. I always finish sand with a fine wet paper, at least 1200 to 1500 grit. Now its time to buff, 3M has a great finishing compound that easily removes scratches and swirls. It is recommended to use an electric or air buffing machine, but if you do not have one a little elbow grease and a few applications will work fine. Remove tape, masking paper and drop cloth and its time to go boating! Next time.... REMEMBER YOUR FENDERS!