Depends if you're going inside the chassis too and up inside the bodywork, minimum of 5 litres, possibly up to 25 to do a thorough job.

Might seem a lot but if you get a quote from the professionals to rustproof the vehicle you'll be looking at between £200 and £450 depending on what you want doing.

I no longer use this product though, having found others that are easier to use with less waste....better too?, time will tell.

As GordonBennett says, it depends if you plan to do inside the sills, crossmembers, box sections etc.

For what it's worth, I would suggest making a few phone calls to classic car restorers / specialists in your area and asking if they will do it for you.

When I had my old MGB V8, a local specialist did all the body inside and out for £70, using a proper compressor and spray wands that could fully atomise the Waxoyl for maximum coverage, AND reach inside all the closed sections.

Admittedly this was 20 years ago but you really need the right tools: aerosols and hand-pump devices just don't get the stuff into closed box sections correctly, and it tends to just 'sit on' surfaces unless either a) painted on or b) properly atomised under pressure.