lps 3 may work to hold rust back but your going to want the block clean no oil or scaly rust and bone dry to mag it first .there is 3 ways i check for cracks . mag powder . if wet the block will not show crack s very good as it will make the fine iron dust in to mud but works good for spot checks . blocks . heads.rods .cranks . i use a wet mag with a fluro oil that glows with black light most all the time now nothing can hide from this check works better then iron powder for over all checks. but if the part has any oil on it will not check as well with the fluro oil used in inspection . penetrant with developer is another way to check for flaws . some rust a check will show thru but clean casting are allways better to see any flaws