Rustproofing underside to ready to paint

Morning all

After a period of inactivity on my restoration on my Campaign - I'm ready to get back on it.

My task now is rustproof the underside, stonechip and colour paint the car on the underside before righting the car. Presently it is laid on its side devoid of all fittings / fuel&brake lines so I've a bare shell to  work with.

So what is the best stuff to use bearing in mind I've cleaned the underside of old seal and loose rust - yes there's surface rust but any other rust has been cut out and new metal put in.

I need to get a good under seal that once dries is paintable. So waxoyl etc isn't what I'm looking for I guess.
Also I've heard that some of the rustproofing products forms moisture traps underneath and can encourage rot to start again

I would like to hear about any products recommendations or  successful experiences.

I do have a compressor so can spray the chosen product if needs be 👍  

First thing is to clean all surface rust as it will further rust under what-ever you use.  I would them spray it with  a good rust convertor/primer that is available now to prep it, as well as a good epoxy primer prior to the undercoat.

Over here we have a Variety of products that you can use  such as Body-Shotz, and Rhino-liner, both dry hard and can be painted over similar to what the factory used.  

When my Golf (81 Diesel 4dr Rabbit LS) was new, I drove it over a freshly oiled road @ 55mph by accident, and that slinging road oil went everywhere… Cleaning it off the paint took about a gallon of Diesel and a lot of clean rags that was in the late summer of 81 (bought it in 80),  my car never rusted at all over the next 20 years of ownership .  

Why body colour the underside? Only the inner arches were body coloured and most of that was just overspray .Are you taking it back to bare metal or just going over whats shown in the photo? The underside was primed, then the rubbery stone chip applied and then the car was painted and assembled. The underside only saw overspray of body colour. Once the car was assembled they black waxed the underside which isn't paintable and used the yellow wax oil stuff around the bay and arches.

I'd be looking at the bilt hamber products. Can't remember which specific ones it is for this job but a quick google will do it. I've used them for rust conversion and bare metal protection as well as the waxy spray in cavity protection stuff. Worth a look. Good luck with getting it done and back on the road!

Thanks for the info information guys. I'm also exploring the Dinitrol range of products and doing some research on those.

Interesting what you said Yomp as I thought I'd read somewhere that the cars were colour painted underneath pre-assembly. As I'm looking at a "stock" finish on the car then more than happy to leave it the black finish and just paint the arches. However one pic attached, behind the fuel tank shows body colour underneath unless this is coincidence and just a white primer??

 Some pics : 

Paint it what ever colour you like, no one will ever see it expect the MOT person.

Just make sure you grind out and treat all that rust as it will just keep spreading underneath the new paint and you won't see it till it's to turns into a hole…

If you are going for a show car/ trailer queen and dry weather car then make it as shiny/colourful as you like but if you intend using it in all weathers then don’t worry to much as it’s good to keep the bottom topped up with waxoyl or similar every couple of years.

Thats just the white ish primer they dipped the bare metal shell into under the fuel tank. It just never got hit with the rubber or the black as its not really exposed to much.
With my last build I literally steam blasted the black wax off which left just the rubbery stuff behind and the above mentioned primer. Any rot was fairly visible and I just cleaned the rubber stuff of locally around the welding repairs and as long as its left to dry properly I just applied the Dinitrol 4941 black wax back to the underside albeit before mechanical assembly as opposed to after.

Heres a pic of part of my floor after the black wax was removed. You can see the red is just overspray from painting the sills and gets thinner towards the middle of the car

Etch primed and then re applied Dinatrol 4941

Never understood why some people go to massive lengths (weeks spent on the their backs on a cold concrete floor with a blow torch etc) removing underseal and sound deadening that is still perfect and stuck like sh1t to a blanket, only to reapply some 3rd party new stuff that won't be anywhere good as the factory original.

I'm with Yomp, just remove the obvious localised bits that have failed or been repaired and leave the rest.

I'm with Yomp and Early-1800, that what I did to my Mk1 Scirocco when I restored that 10 years ago and the floor is still rust free.
My Mk1 Golf daily drive is still wearing its original floor covering at that lives outside, used in all weathers and nearly covered 300,000 miles, I just check the floor, wheel arches etc  before and after winter, treat/paint any rust/stone chip damage and top up waxoyl when the weather is nice and warm every couple of years.

It's different if you want a show car with mirrors underneath it at shows….

Say the saying goes…. "if it ain't broke don't fix it….."  

Thanks for the pics Yomp - hope mine comes out like that!😄 - given me something to aim for 👍

I'm certainly leaving the factory applied underseal thats still solid. The odd patch has "blown" which I've scraped off to an area where it's solid again. 

I'm going with the dinitrol rust converter then primer application and then dinitrol products plus the cavity injection in the sills and box sections. 

I'll colour paint the arches obviously and then it's a case of reassembly (at last) 😉

I would leave the factory stuff in place wherever possible, just remove rust locally, nothing aftermarket compares to what the factory used.

I primered bare metal areas and used a U-pol product called in the UK ‘Raptor Liner’ , it’s a two pack product designed for coating the load bed on pick up trucks and it’s very durable.

Follow that with a good wax based product which stays soft and is not over paintable, but is anti- corrosive


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