Never used rustbuster 421 but have had plenty of experience in foam rollers..........with oil based gloss though
I dont know the consistency of rustbuster once mixed but if you can get it the same as oil based paint to around 5% thinned i would go for any high density foam roller
Screwfix do some,the skinny ones are best,fat ones holds to much
The trick is fully load them with paint(but not saturated) this removes the chance of creating air bubbles,dont apply to much pressure let the roller do the work.
Dont know how big an area your working or what finish you want but always go over a wet edge
Hope this helps
They aren't all the same. I use foam rollers for rolling blue out on my surface plate but the last lot I got were total rubbish, depositing fluff on the plate as the foam disintegrated right from the off.