Have a look on the Machine Mart Website. Product code 000512105. Its an underbody sealing gun but it has a hose that can go on the end of the gun for spraying inside things. Used one for my Land Rover chassis. From memory I extended the hose to get well down the chassis rails.

Martin C

Choose your pipe, make a bung to fit in the end, drill the bung with a small hole, cut a groove across the front of the hole. Fit bung into end of pipe and spray. The spray pattern will depend on the flow rate, pressure, size of hole and shape of groove. A little experimenting should give you what you want.

I had one. It was a piece of clear pvc tube with a wood screw in the end, and a few pin holes. The spray pattern was unlike any other spray nozzle I've come across, but it sort of worked for Waxoyl provided it was warmed up or diluted with white spirit.

Get a Cap head screw say M8 about 50mm long. Drill right through from the head to the end about 3mm. Then select a self tapper, 5mm is good. Cut a length of this say 5 to 6mm long. This is your swirler. Now drill through the cap sure again to 5mm leaving about 5mm of the original 3mm bore. Press the piece of self tapper in as far as it’ll go.

Now you can fix the screw, head end first in your hose. The swirler will cause the pressurised fluid to have a tangential velocity component. This will result in a finely atomised air cored cone spray from the 3mm hole.

There is a lot of physics behind this. I’ve researched it Un depth as use for rocket engine injectors on my blog.

Why do you need air? You can get airblast atomisers but in the scheme I’ve outlined the pressure atomises the liquid acid. Actually instead of doing what I said you could just get a garden sprayer, whose nozzle works like I described. Put the very end of the nozzle on a piece of flexible tube. Fill your sprayer and pump away.

1cm x 1cm box section is way too small for any off the shelf wax sprayer to deliver a suitable thickness coating The typical recommended coating thickness for cavity wax is around 0.1mm 'wet' which reduces to about 30% of that when all the solvents have flashed off. Underbody waxes are applied at about 0.75mm flashing off to about 0.4mm.

By off the shelf wax sprayers I include the waxoyl pump-up tin of old, the aerosols with extensions, a Schutz Gun and the professional compresser fed cavity wax gun with multiple interchageable nozzles and spray patterns as used by many body shops.

All you will be able to do with any of those methods and that size cavity is to massively overtreat and partly fill the cavity and that can stop it draining. This could be a very bad thing to do, actually promoting more rust and making any subsequent weld repair 'interesting'

It's the wrong time of year to be considering treatment, pick a very hot day and get the wax as hot as you can. Let any excess drain, maybe drilling additional holes Even then you'd be far better off fabricating a new piece of steel, possibly a bigger section if you can, properly removing the rot and getting it properly welded.

Dinitrol do an aerosol phosphoric acid RC900 which can (although not specified as such) use an extension nozzle used for their range of waxes but it will only give you a couple of feet of extension.

rejelrustproofing on ebay is a good mail order source for Dinitrol phosphoric acid, cavity and underbody wax and application kit (no connection)

While it is British I'd not use Waxoyl, it's still iirc a one mix formula does all and cavities have totally different requirements to exposed underbodies, and old cavities have different requirements to new ones. The waxes that vehicle manufacturers use have been way ahead of Waxoyl for decades.

It sounds as if you access the ends of the box section?

If so, Waxoyl, warmed, and sprayed from a paraffin sprayer with a long spout, powered by an air compressor set to deliver at least 80 psi, should be finely atomised, and will coat the inside of the box section pretty thoroughly, especially if the nozzle can be inserted fully from both ends.

I used this when we had a Renault 6 and 16. The only sections that rusted, as you might guess, were the ones that I could access to spray. Their successor Renault 5s, treated similarly, had not rusted after 17 years, and one lived outside!

Do warm the Waxoyl can in a bucket, or sink of really hot water, before filling the sprayer, that way you should get a real mist that penetrates well.

Howard