Like all jobs well done, 90% of the job is in the preparation, and the rest is usually getting the basics right.
prepare the surface to be re gel coated by sanding it back to good clean original material, mask off the areas that you don't want to be covered with the new layer, wipe the area to be covered down with a rag wet with acetone ( this will clean the area of any dust, "oil", moisture, and actually "prepare" the old surface to best accept the new gel coat) It is often a good measure to apply a thin layer of "bog"over the area before you get to any gel coating, then fair it back with 240 grit dry paper this is to fill any pin holes, particularly if your back to actual exposed glass fibres. (always wipe with acetone before any layer is applied whether bog, resin, or gel coat) When it has been faired then masked and wiped down mix your gelcoat with a "small" amount of "wax in styrene" which it is best to purchase already mixed from a fibreglass supplier (or boat builder). It should be 5% wax in styrene, and a very small amount goes a long way. Better still, if you purchase, instead of just gel coat, "flo coat" which is already mixed for external (out of the mould) gel coating, this will leave your finished surface of the gel coat hard and non "sticky" when it has fully gel'd, and you can then fair it with sand paper and wet and dry with out the paper clogging up with every stroke. The MOST IMPOTANT THING ABOUT MIXING GEL COAT is that you NEVER NEVER add more than 2% catalyst to your gel coat/resin, as to increase the catalyst (incorectly called hardener) does several nasty things that you don't want. as a larger percentace of catalyst is added the chemical reaction becomes more violent, more heat is produced, the polymer chains formed are smaller and smaller to the extreme that the mix will smoke or the finished product is almost "chrystaline" but whatever the result the finish gel coat is appreciably weakened and at times, over catalysing will actually leave what appears to be "unset" areas of gel coat/resin. If you are working in cold temperatures, never increase the percentage of catalyst, but after applying the gel coat raise the temp[erature of the air around the job with heat lights,or blow heater etc. If you use the correct catalyst percentage mix, MIXED WELL with the gelcoat, ie stir for up to a minute or two, after aplying, when the temperature around the job is above about 20 degrees celcius, the gel coat will gel in 45 to 60 minutes, give it about another hour at that temperature and you can then fair it back and cut and polish it to finish.
Don't try to put it on "thin" as that is not how gel coat is meant to be used, and it is very poor practice to "thin" it with acetone, (if you have to thin it, thin it with "styrene monomer") it is not like "painting" you cannot finish "off the gun" you have to be prepared to shape and fair back by hand. If really thin layers of gel coat are applied they become more succeptable to irregular settings. The volume of gel coat is so small that the required chemical reaction will slow down dramitically and in some instances not apparently go off at all, or if the ventilation is poor the whole thing can "tripe". If the application is done correctly polyester is a very easy material to work with, and will not need any thing applied "on top" of it to cure it, in fact by applying PVA over wet gel coat you suffer the great risk of breaking down the catalyst in the gel coat by the intrusion of the "wet" PVA
