I am going to assume that where the rust appears now was already rusty before you started to work on it. Maybe not bad enough to really see but it was there. To stop rust(oxidation) you have to 1.remove oxygen or 2. Remove moisture. In your case I think what happened was with the copper blasting was not aggressive enough to actually remove metal to get to the bottom of the rust pits and remove the rust. And your sander only smoothed off the top not removing those pits. Those little pits still have rust in them and absorb moisture and oxygen. When you then coat the surface with penetrol it might block further oxygen and moisture from getting to the surface but does not remove the the moisture and oxygen in those tiny rust pits and they keep oxidizing even though they are sealed from the elements. In the body shop industry the only way to fix rust is to replace the rusty metal. The next best "fix" is to aggressively grind the area to remove all those tiny rust pits. 40 grit on a grinder. 80 grit on an orbital is not aggressive enough. And those pits can go more than half way through the metal. I work in a body shop and to really get those out takes alot of careful attention(magnifing glasses and a good light) to get all the pits. Now you have to remove those aggressive grinder marks(alot of work) to get it to look like the un rusted areas. Then apply the top coat for protection. Then on the other hand. If the metal was perfect, you have you be faster between sanding and top coating.