There is a thing about Rodinal that is not always appreciated. It has a limited contrast index with HP5+ and perhaps with FP4+ as well at higher dilutions. I could not get any more than 0.61 out of HP5+. Extending development may build up density, but it may also cause grain clumping which leads to excessive grain. I am not referring to stand development, which if done well is not extended development. The upshot may be a fine looking negative as far as density is concerned, but a grainy one. This is a suggestion, not a pontification. It is something to look out for. If that is what is causing the excess grain on FP4+, reducing development time will help, and wont have much effect on contrast.