I have been advised that I should apply Ospho Rust Preventative after stripping old paint off a frame, before I have it primed and painted. I would like to hear about other people's experience with this product.

I've used it on bare steel as an etchant, but I think its primary use is for treating rusty steel. I understand that it can be used prior to finish-grade painting, but I've only used it on heavy duty stuff like car frames & suspension parts. A few tips:

If there is any rust on the surface, the Ospho will tend to form small bubbles which harden when it dries, so you may need to sand again just before painting. I think that may be necessary anyway, but I don't recall exactly.

Be sure to check with the paint shop before you apply it, it may interfere with their process.

Wear old clothes or a disposable coverall when you use it, it's very thin and you'll get little droplets of it on your clothes which you won't notice until the fabric dissolves (it's pretty caustic...)

I've always heard Boeshield T-9 was the product to use for rust preventative frame treatment...

You can't paint over T-9, it's used for inside the frame tubes. I assumed vonfilm is considering the Ospho as a pre-primer etchant, it's meant to be topcoated.

Reading about Boeshield T-9 on their website it does not sound like it is a permanent treatment suitable to paint over. They say it leaves a waxy coating that will last for many months. I am planning on stripping a bike frame with paint stripper before having it primed and painted at a auto body shop. I am afraid the naked steel will oxidize in the air before I get it to the shop for painting and priming.

What pre-painting rust preventatives do other people use?

I have used Ospho extensively on ships. It is an acid, so definitely wear protective clothing, goggles, whatever. It will convert rust to iron phosphate, or in english it turns brown rust to black, theoretically intert and will not continue to rust. It leaves a light film which the manufacturer says that you can paint right over. In my experience using advanced paint systems (expensive paint), you shouldn't have anything there that it doesn't recommend on the instructions. So what I've done in the past is apply ospho, and then right before painting you can clean it off with a mild solvent like paint thinner. Then you get the rust removal action of the oshpo, without the residue that may or may not get along with your new paint.