about 13 years now when i was painting cars /semi,s big stuff full time and part time air planes. i did some very basic testing of etch primers and epoxys primer . variprime came out one of them on top so there not many epoxys that lock in as the etch is it a acid etch . i would not pass it by for lets say PPg DP just will not stick as good to plane steel and no way close on alum i have paint jobs out there that are getting to 20 years old were i use varprime let the thinner gas out for a day or two then a lite sand then Dp 40 then K2