ok, just got yer email. the finned areas go all the way through in some spots. I masked mine just like yours with gorilla tape for blasting and when I was done it was full of alum oxide(blast media).
it got in through those passages. so when I painted it I stuffed rags in the spark pockets. and there is another area in the middle that goes all the way through to the valve side. you won't have much to worry about it because it prob won't get that far in. I would mask the spots where the exhaust screws go in, the whole flat spot around the screw hole, otherwise it just gets scraped off, but it's no biggie.
plastic is always a problem cuz paint drys on it fast, then when it moves it crumbles off onto your paint job. paper or just tape like you did is the best.
tips: you don't want to build up too much paint on the head. but to get in all the small areas a lot of people end up going too far and getting runs, etc. what you need to do is; do one light coat(the tack coat)- just mist from about 12 inches all over. this is a very layered surface so you need to do your passes aiming up slightly, then down slightly, then to the left a lil, then right the same. they have to be light or you will get runs. if you go all in one direction you will have holidays.
so 1st it's a light coat not complete coverage, only about 10-20%, so you still see gray.
let that dry for 10-15 min. then do what will become a complete coat. from about 6 inches. to avoid runs, get the deep areas first, all the little weird crap that you have to get from different angles. do the obvious broad strokes last. try for uniform coverage.
you can stop there, or wait 10-15 min and do one more coat, but if you're last coat is a light coat, some paints will flash a bit. so a full coat to finish is best.
everytime you stop for more than 10 mins turn the can upside down and spray til it's clear, and when you start again start your initial pass off the piece, so if it spits out a boog it's on the wall, not on yer paint job.
