Black Moon,
Now that everyone has finished bashing Gelcoat as incorrect, I will give you my opinion on it. I have used the Ecklers gelcoat in the past and will use it in the future. I have also used Slick Sand and it's predecessor Feather Fill on cars for many years, and find all of them to be an acceptable base for any type of finish system. That being said, each material has it's advantages and disadvantages.
First you have to understand that both materails are polyester resin and have more in common than most people are aware. The Slick Sand has a lot of inert filler material in it so that it will build up to a high film thickness to allow you to block out even the roughest panels. This material is nothing but Body filler in a sprayable form! Plastic body filler is the exact composition and uses the same filler material. You could accomplish the same effect by skimming the entire body with Bondo and then sanding it smooth. The gelcoat however, is pure polyester resin with a small ammount of a waxy material that forms a skin over it when sprayed to help it level better. This composition does not make gelcoat a good product to level rough panels, but makes it an extremely good product to seal old repairs and to keep old spiderweb cracks from coming to the surface. Now for the downside. Gelcoat is miserable stuff to sand! The wax that forms on top of it will clog sandpaper and even after the wax is completely off, it is hard stuff to sand since it is so dense. This same hard, dense surface is exactly what makes it , in my opinion, a great product to use. Gelcoat does not build to a high film layer like Slick Sand and cannot be used for blocking panels.
There is no doubt in my mind, that gelcoat will form a harder less permeable layer than Slicksand, and if you do not belive so, than just strip the paint off both products! If you use the common Methelyene Chloride stripper on a car that has used Slick Sand, the stripper will attack the material. The only reason for this is that it is attacking the inert material and not the resin. The material is so light and porous, that the stripper can penetrate and attack the filler, just like body filler! Gelcoat does not have this problem since it is nothing but resin and almost impervious to such chemicals.
Gelcoat, in my opinion, does a better job getting down into small spiderweb cracks and consolidating them to better prevent them from showing back through. I also think that it does a better job of preventing swelling of old repairs under lacquer topcoats, since it does a better job of preventing the lacquer thinner from penetrating to the repairs. This obviously, is of little concern if you are using a modern urethane finish, but is important for those of us who still shoot lacquer on judged cars.
In closing, I will tell you that either product will give you good results, especially under urethane, and there is no doubt that Gelcoat is harder to work with. You will probably have to use a surfacer material even if you used gelcoat, while Slick Sand is a single product. A lot of it boils down to whether you are doing the job yourself or paying someone to do it, and what your expectations are. I do all my own work, and it is nothing but time to use Gelcoat, but If I were paying someone to perform the extra work, I might have second thoughts about whether it was woth the additional cost.
Sorry for rambling on so long!
Regards, John McGraw
