I have that china freight air descaler and they work great.

Once Por-15, KB seal are exposed to the atmosphere they will not last long. I used to buy the smallest cans and just use them up at one time.

The sheet metal screw with rubber washer will work for a little while, but I wouldn't expect a 90% used can to be good in a year.. However, a few weeks, or so, I can see that working.

This is the one I have
Air Needle Scaler & Other Needle Scalers - Harbor Freight Tools

That baby is heavy. The compact one i am sure would be just as good and smaller so you can get into more places

Compact Air Needle Scaler


You should be good though as long as you don't have any rot through areas. You get that outside coated really good with the POR 15. Then clean the inside of the frame and shot it a heavy coat of fluid film. Seal all the frame holes off with even silicon. Then coat the outside of the frame good with FF. You should pretty much super slow down the rust progression..