As has been asked....did you wait a period before painting the treated lumber? Because of the treatment process, large amount of water are introduced into the lumber(AKA...Presuure Treated) as a carrier for the salts and other chemicals. Most of the time the wood is not kiln dried afterward, unless you pay for the really good stuff. Most treatment plants let it air dry until the moisture level makes so the lumber is fairly stable. Even then the moisture content is higher than conventional framing lumber. This is why most treated lumber is heavier and more prone to go crooked when not nailed down or stored properly. This moisture is why paints, stains and other finishes may not properly adhere if the lumber has not been allowed to dry. The moisture from within, trying to escape as vapor will bubble paints because paint is not porous enough to let water vapor escape. Same goes for Spar and other varnishes. This is why most recommend waiting a coupla months after exposure to the elements before painting/staining/sealing. Wood expands/contracts from moisture/humidity and temperatures and this lead to cracking of finishes that form a solid surface. As for what to use....it probably doesn't matter. Treatments like Thompson's, that soak in and penetrate, need to be reapplied if the wood is in direct sunlight every coupla years minimally. Spar varnish(even tho it's used on ships) also need to be periodically reapplied....more so than good paint. The secret to getting paint to stick is to use clean lumber and to make sure it is primed properly. For the most part, treated lumber used for something like a shooting bench does not need a finish to protect it. It is more of a appearance kind of thing. There are also newer products out there such as Behr Deckover, Rust-oleum Deck Restore and Olympic Rescue. These are just a really thick paint with a high percentage of solids to fill cracks/splits. The high percentage of solids also give traction and slip resistance by creating a rough texture. They react similarly to the outdoor exposure as paint does and life depends on the condition of the previous finish and prep work. New one out called Gulf Synthetics DeckRevive, that is supposed to create a porous film than while protecting wood from rain, allows vapor to escape(similar to Gore-tex fabric.) Pretty expensive stuff tho.
My suggestion would be..... use a orbital sander to remove as much of the old loose/peeling paint as you can from the surface, then use a good quality primer and a good quality paint. If at some point yopu want something else, just replace those few boards that you used for the top itself. My shooting benches are all made from treated lumber and have treated plywood tops. I have yet to use anything on them at all. While I do store them under cover when not actively using them, they are exposed to the elements.
