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MG MGB Technical - Factory Underside Finish

Hi All,

I'm preparing to get working on my restoration after a long break. I'm planning to:
(1) grit blast the underside to remove underseal, paint and minor rust,
(2) apply, by brush, a zinc-rich primer (around 95% Zn when dry) as cold galvanisation cathodic rust protection (similar to Galvfroid's offering) over metal cleaned with thinners.

The zinc-rich primer is single-pack so I've been advised by the salesman at that two-pack or even cellulose paint are not compatible as paint overcoat treatments.

So, I'm looking at either:
(a) a polyurethane gloss paint finish (Polyguard QDP, again by,
(b) paint finish over MIPA underbody schutz underseal.

Can anyone offer advice or indicate their experience with these two (or alternative) approaches?

Does anyone know how the car underside was finished originally? Did they have underbody seal in 1967?

Many thanks in advance,
Brian McIlvenna

As far as I know, in 1967 the underside of the car would have had a brief spray of the same paint as the top-side of the car. There was no thought of protecting it from grit, stones, salt or water. That's why they rusted so well! Underbody seal was available in accessory shops for the fastidious owner to apply. It was a sort of rubberised product that deteriorated with age and lifted, allowing water to get between the coating and the steel.
Mike Howlett

My '67 was finished in brown primer, followed by underseal.

The underseal was even on the boot floor, above the petrol tank.

There was no body colour paint, whatsoever.

Several other people have confirmed that their early cars were similarly finished.
Dave O'Neill 2

"allowing water to get between the coating and the steel."

After which it corroded faster than if it hadn't been protected!
Paul Hunt

I recall seeing photo's of later car bodies (B/GT's) being submerged in a vat of primer hung from a conveyer system to insure complete coverage. That said our '74 B/GT "appears" to have under-seal applied over the primer coat, and the finish coat of paint (top & bottom) over that. Way too cold for me to go out to the garage to verify.
Larry C '69 Midget

"I recall seeing photo's of later car bodies (B/GT's) being submerged in a vat of primer hung from a conveyer system to insure complete coverage"

I visited Longbridge in the 70s and that's what they were doing. The Mini was particularly amusing, they put a 'spit' through the front and rear bulkheads and rolled them through the vat sideways.

They also had engines on a conveyor being spun mechanically, no sumps as I recall so the pickups were in a long bath. Also forging crankshafts from plain billet in three bangs, and induction heating the oil pump and distributor gears to yellow hot in a few seconds.
Paul Hunt

When I stripped the undercoating off my rear fenderwells, All we could identify was the pink primer and then undercoating. There was a coat of paint over the undercoating.
Bruce Cunha


Thanks for your input.

Would you have any idea what type of paint is used for the MG body panel factory coating?

I want to remove this on the panel underside areas so as to apply zinc-rich paint. I was thinking standard cellulose thinners may remove the factory paint.

What I'm planning to do for metal protection then is:

(1) Grit blast the underside with a fine-medium sodium bicarbonate medium. I understand this is is a non-aggressive process.
(2) Apply by brush two coats of a cold galvanisation zinc-rich primer; a treatment very close to the Galvafroid product (90-95% zinc content when dry)
(3) Apply a bituminous underbody schutz, possibly by air gun if I get around by buying an air compressor, purpose gun and air hoses.
(4) Apply a tough polyurethance satin paint over the schutz; the colour can't be mixed to the car's colour but is available off-the-shelf in a colour close to the car finish.

Thanks again,
Brian McIlvenna

I seem to recall Clausager saying somewhere acrylic was used from fairly early on, then others (polyurethane?), so cellulose not used for very long. However mine was repainted (for the fifth or six time from what I found when cutting it back) in cellulose in 1995, then there's two-pack, base coat and colour coat, and possibly even water-based, so it could be anything now.
Paul Hunt

This thread was discussed between 02/02/2015 and 10/02/2015

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