With even a reasonable cleaning the glyptol should adhere well enough that it shouldn't be a concern. The stuff is tough, originally made to be used on electric motors which can get pretty hot! I've seen blocks come out of the hot tank and the glyptol remained completely untouched.

I've always used the spray can stuff - I hate brush-painting anything! I used corks to plug the lifter bores and I only bothered doing the valley, not the rest of the block.

I doubt you'd see even a minute HP gain from this since all it supposedly does it provide a smooth surface for the oil to flow down. You'd have to do some other mods to keep it away from the rotating assembly before you'd see HP gains.

I stopped using the stuff over 20 years ago. With the tools available now I can quickly get into every part of a block and knock out every bit of flash or casting crud and I'm happy with that.