Originally Posted by sailvayu
The surface seal is a wax you put in at about 3% when you are using the gel coat out side of a mold. You mix it into the gel coat prior to applying. The wax will rise to the surface of the gel coat as it cures to seal it from oxygen. This will give it a hard surface. Without it the gel coat will cure with a sticky film on the surface that makes it hard to sand and will result in a soft surface. Add it prior to the hardener. You could just add it to the whole quart as it does not sound like you will be doing any mold laminating.
+1 mostly. Poly resin and gel coat are air inhibited, which means the surface layer will not cure when exposed to air. This is so that you can laminate to it with no prep and get a chemical bond. To get a surface cure, you need a surface seal. There are many methods of doing this, from PVA to wax surface seal. If you are not doing a molded part, and therefore want surface cured gel, you need to mix in the surface seal at 2oz. Per qt. I often use slightly less. Do NOT add it to the whole batch. The proper method is to spray 2-3 coats catalyzed with no wax, allowing 15 min to half hour between coats depending on temps, and finish with a final complete coverage flow coat of gel with wax. The wax will rise to the surface, creating a surface seal that will allow surface curing. If you have wax in all coats, it will rise to the surface at each stage in between coats, creating some cryptic problems you will not be able to explain. Pre waxed gel is for amateurs who don't know better. This is the reason they sent it to you separate instead of sending you a whole can of pre waxed. If its at all cold there, make sure to heat up the surface seal before mixing. This liquifies the paraffin wax in styrene monomer which makes up surface seal. If you don't do this you'll get little chunks of wax in your finish that won't want to polish out. We used to make our own all the time back in the day. Many people prefer PVA, but not me. Properly used surface seal is superior, though more technically challenging.