The original idea was to thin the rustoleum to the consistency of milk. That requires about a 40% thinning. It is thin but the idea, I believe, is to apply a thin coat of thin paint. So if it runs, it's not necessarily to thin but it may be that you are putting on too much paint at one time.

I think that thinning to about 40% is required and then wring out roller and apply a thin coat.