Opti coat over Spar?

[ray6]

Spar (and most other solvent based coatings) require a 2 step curing process. The first, sometimes called flash or flashing, is the evaporation of the solvents. In the case of spar/MS, it's the MS. This also means the more MS in your mix, the longer the flash time will be and the lower the solids content and shorter lifetime.

I don't think that the spar method will give you 2 years, with or without OptiCoat. I think you'd need a better coating than spar to get 2 years, probably 2K water based and 2 coats.

Once the coating has flashed, the second step begins; with polyurethane (spar) coatings, it is by oxygen adsorption. The oxygen combines with the coating.

You can shorten the first step by increasing the evaporation rate of the solvents with heat, moving air, etc., but there's a limit. You run the risk of skinning, which is the process of forming a sealing top coat with solvents trapped underneath. This is why coatings requiring external energy to cure like IR or UV, will warn you not to cure until coating has flashed.
ray6