Before pictures Inside cowl I sand blasted. If I had to do it again I would use a pump sprayer and muriatic acid. Let set for a time Depending on your situation. like noting painted yet. test an area and observe results that you want. Rinse with water and repeat to get the maximum results. After last rinse before completely dry apply SEM Rust mort. Will prevent flash rust. Read the label of all products!
The inside of the cowl was never originally painted as far as I know.
Factory paint doesn't like to come off with today's not as good EPA friendly paint strippers. This was an entire day of applying, scrapping and power washing off and still couldn't get it all to even want to blister.
Motorcycle tank.1st picture rust inside tank poured muriatic acid in and an immediate reaction.2nd picture 2 hours later. and then rinsed with water.Then put SEM Rust mort in and dumped and rinsed again. 3rd picture, to keep tank from flash rusting. put WD 40 in and dumped. rinsed with denatured alcohol. 4th picture Allowed to dry and installed Red Kote. There again read all caution labels. It does work! Be patient and careful. I left the petcock and plastic filter on. The muriatic acid ate the filter. no matter was getting a new one. Do not use Evaporust.
The best way is to drill a bunch of spot welds and remove the cowl. That way you can find all the rust holes you didnt know was there. The only paint these got inside the cowl was a little overspray that went in through the grill screen area.
Without drilling out the spotwelds and completely disassembling the firewall and cowl a good way to get in and blast or treat is to have one or more cowl patch panel area you need to address to start with. Then along with the two interior air vent ports and cowl grille openings you can really get tools and nozzles around in there. My vert had patch areas driver and pass side that gave great access.
I started by sanding the inner cowl area to loosen up any surface rust. I then sprayed Eastwood Rust Encapsulator in the cowl area through the grill openings and side drain holes using several coats. I then spayed a light coat of internal frame coating paint till it came out the drain holes.
I was lucky enough to not have any rust through in this area. My inside cowl area had a mix of overspray factory paint and surface rust. I especially coated the raised lip area around the openings by the vents and leading to the heater box. Just wanted the area to have maximum protection. (Before and After pics)
Here is another example of muriatic acid removing rust. Plate was bent up by an excavator removing it. Straighten what I could and made a back splash. Laid rags on top poured muriatic acid on top.let sit over night. Presto! the hard work is done. The back splash is what it looked like. not going to paint so I only put WD 40 on. It's just a work bench.