Here's the deal on thinning resin.
Up front it sounds like a really good idea to reduce the viscosity so it will penetrate quicker and further into the wood, but the chemistry doesn't help you get there. Unlike a paint where the solvents evaporate out of the mix and leave the solids, most everything in the resin stays in the mix and is supposed to cross link and become a solid. As you add solvents they tend to contaminate the mix, acetone is an inhibitor that prevents it from cross linking, and styrene is only useful up to a certain %, past that is degrades the resin. When polyester resin is first cooked it's almost a solid at room temperature, so they mix in styrene to reduce the viscosity, styrene also cross links with the other raw materials in the mix and actually helps the cure take place, they add the right amount of styrene at that time to achieve the best physical properties, and this includes water resistance.
This blend now has a very low viscosity, too low for use as a laminating resin, so they add silica to thicken it so it won't drain out of the glass while it cures. Now when someone adds more styrene to thin it back down you're battling the silica and trying to reverse its function. This added styrene cross links with itself and isn't nearly as strong or water resistant as the resin was to start with. Acetone inhibits the cure and slows the process down, but doesn't cross link, so it needs to evaporate out, which it can have a tough time doing, so much of it gets trapped in the resin. It typically finds its way out over time, but now you have weak and under cured resin that has poor water resistance.
In a boat shop where speed of construction is important, they frequently don't pre wet the wood, they just lay glass on it, the wood may suck resin out of the glass and leave both laminate and wood starved for resin, this is where the poor bond comes from. Pre wetting the wood and letting it at least start to get hard before applying glass gives a much better and longer lasting product. Now if you really want to have better penetration you can buy a low viscosity infusion (method using fiberglass) resin, it won't have any silica added to thicken it up, so you won't need to add any solvents.
