I've been spot sandblasting my undercarriage for years on all my rigs. It does create a huge mess, is time consuming, etc. But it keeps the rust at bay. I'll be the first to admit the time needed to keep up on this is probably not worth it.

I've never seen an overlay product that truly stops the rust. They simpy slow the process. They all will fail over time. Of course the definition of 'time' is important. If you're looking for a couple years of protection, the better products (POR, Rino, etc) work very well. If you're looking for a permanant fix, an overlay isn't it .

What I now do is;
1) Use a medium course sand
2) Blast area to absolutely pristine metal
3) Clean with cheesecloth or similar
4) Use 3 to 5 coats of 'clear' POR-15.
5) Top coat with a clear non-UV reactive enamel, etc

The trick above is using clear. The reason I do this is if rust reappears, I can see it well before it gets bad enough to lift/bubble the paint. This way I can hit it again before it becomes a sizable issue.

Warning - Using sand as a media can be extremely danagerous. You're at high risk for developing 'silicosis' which is a very nasty lung diease. Unless you have access to the proper equipment, I'd suggest using another less dangerous (but more expensive) blast media.

Hope this helps
Kent