Nothing which is usually done made me happy, yet I could not come up with anything better. Without going into very much detail as I don’t want to bore everyone, here are the brief outlines of my findings.
Disadvantages of common underside systems.
Underseal: There are two main types of this. The stuff which sets hard-ish, and the stuff which stays sticky. Both are absolutely awful to remove from a car, trust me. So once you have a newly-repaired underside, there was no way I was going to cover it in ming again. Also, you cannot see what is going on. It’s all hidden under dreadful black stuff, how do you know the spot welds holding the chassis rails on aren’t rotting out? The sticky stuff is not easy to keep clean either. And the stuff which sets is worse than useless. As I have seen time and again on many cars, It traps water underneath and accelerates corrosion. As I found out and mentioned much earlier on when I was removing underseal, there are areas which ad been wrecked by the underseal, water collecting underneath it rotted the old footwell/floor pan/transmission tunnel area. Also, repairs from about 5 years previously, “”protected”” by paint and underseal, had fared worse in many cases than the original 1972 metal.
Stonechip: The older sorts of stonechip trap water underneath them. Like underseal, they were (and some types still are) a liability. The idea is to have a flexible layer of stonechip, with paint over the top. Hang on, how does this work, a tough layer of paint floating on a flexible surface? Surely this needs very flexible paint to be effective. Again, it hides what is actually protection the steel.
Paint: Just paint underneath? Hmmm, an effective primer then tough 2-Pack over the top might almost work, maybe for the types that only bring their cars out on dry days for a trundle around the local villages. It just doesn’t have the toughness to withstand a battering from stones, while it might just about work for some people’s cars, it won’t work on mine, which will get used and ragged.
Oh and underwing shields: Do I even need to mention these? Well suppliers keep listing them, so I presume people are still buying them? Well yes, they protect the paint from stones. But you are also creating a double thickness inside your wheel arch, and the accumulated muck and water is going to accelerate corrosion like nothing else apart from driving up Bristol channel every day.
A couple of the most important things learnt were that the flexibility of paint used underneath is not of paramount importance. Yes body shells do flex (A lot more than you’d ever imagine) however the amount of movement in a given area is far less than the upper limit of flexibility of most paints. In other words, the steel flexes very slightly, most paints flex a lot more.
The other thing is where exactly corrosion starts. Yes, if the coating chips of flat panels, they will start to rust. However, about 85-90% of corrosion on these cars starts off at the joins. Yes, those double thicknesses of any sorts, where two panels are spot welded together and moisture gets in the flange. This is where rusting starts. And once it has eaten away the flange, it starts out dissolving the panel. And it happens faster underneath as this is where most salt and water is, although of course anywhere where air can get to will rust as well, so nowhere is safe on a body shell. So the flanges underneath are the most prone to corroding and are the biggest hazard. This is where seam sealing plays an important part.
Then came some interesting correspondence with Robin Gell, who was having the similar thoughts about how to protect the underside of his Factory V8. And I do believe he has hit on it. I was a bit apprehensive about “copying” someone in this way, in case it seemed I was following something unproven or taking someone’s word without giving it enough thought. But no, after hours of blahhing on about paints with various classic car restorers, those in the trade, and plenty of sleepless nights, I decided this was the way to go. More than worth a try, it seems far better than anything commonly used.
Over bare steel, it works as follows:
Bondaprimer. This is zinc-resin primer, which apart from getting to your head very quickly, is a fantastic primer for steel. The resin formula means the adhesion to steel is excellent, indeed trial pieces left to harden for long enough showed impressive resistance to battering with hard pointy things. High zinc content means corrosion resistance is extremely good, I have been shown steel fabrications made over 20 years ago which have been treated with BondaPrimer and kept outside, and when I was given permission to remove small areas for inspection of the steel (as long as I repainted said areas myself!) the metal looked fresh like it had only just been overpainted, on flat areas and over joints as well. This stuff has been used by a local garage for many years to good effect. It seems the best primer for underneath, it is very easily applied by brush and while it does not have as high a zinc content as the Electrox I used in the box sections, then smooth finish and similar corrosion resistance make it ideal.
Jotun HF Marine 2-part gloss: This is a real test as to whether Stephen actually reads this, if so he will make a lame quip about MGBs handling like boats. Paint designed for use on the superstructure of boats, the resistance to road salt should be good then! It is highly resistant to water, a very tough 2-pack which looks good even when brushed on, and will protect the primer from damage as well as giving the underside the correct colour as it can be had in a large number of colours, one of which will be near enough for the underside. Testing on scrap steel, over Bonda primer, again shows great adhesion and chip resistance, I am confident that this very high quality paint will last a very long time.
Plasti-Dip Soft Coat: This is going on over the top. To act as a sort of underseal, but not quite. While the Jotun HF is very tough paint, it still really needs another coating over it to protect from damage by stones. I would be very unhappy just having the paint underneath, it needs something to protect it. Plasti-Dip is a solvent based rubber, which can be applied by brush. It dries quickly so layers can be built up to give a thick coating in the areas which need impact resistance. The best part is that clear Plasti Dip can be bought, which means the white painted underside will still be visible, the condition of the paint can very easily be monitored incase any area are starting to corrode, and the condition of the Plasti-Dip layer is very obvious so it can be recoated easily if any areas suffer worse than others. All in all this layer will protect the paints without looking awful or hiding the condition of the underside.
Ok then, enough wordy stuff, loooooook!
The first coat of Bondaprimer drying. To coat the underside is a lot of work, I started about 8pm and was out there until half two. Blarrrgh those zinc fumes! What a relief to have everything protected from flash rust, and looking rather nice if I say so myself.
Seam sealing! The next important stage. And I know it looks a mess, but it's all smoothed and blended in. The trick is to dip your finger in white spirit so the seam sealer does not stick, then it can be spread and smoothed easily. Everything was sealed underneath, to block up any microscopic pores in my seam welds, and to seal the few remaining original spot welded flanges. No air or water is getting to the metal now!
More seam sealer, and a bit more to do. I spent a lot of time and attention to detail doing this, bearing in mind the yearly floods around here sh will have to cope with.
And then it was all covered in the second coat of BondaPrimer. I'm glad I diddn't spend ages grinding down all the plug welds, it would have made little difference really. And yes, after all, this is the underside!
I am confident this stuff will protect my new floors, sills, crossmember, chassis rails..... Spare ROStyle holding things up! Lovely thin well spreading primer, it gets into gaps very well.
And the first coat of 2-pack! Does that make you MOIST? Yes I'm glad I've saved that (admittedly innapropriate) word until now, it's the only one to describe the smooth, glossy, pale white underside. The Jotun HF is a joy to use, creamy and easily brushed on in a thick enough layer. Rememeber this is one coat of gloss white over deep red! I have to admit, brushing the white over dark red, I can see the attraction of the Works MGBs. However, I would have a darker red with the white if I ever make a works-ish car!
With the second coat of Jotun HF on, she's now looking like she's worth something. No longer lots of panels melted together by a mad man with a MIG, but a proper bodyshell which will soon become a car!
And there she is after the first coat of Plasti-Dip. It is very painful applying this matt-finish rubber over the beautiful gloss. Most depressing. Arghhhh! But even so, it still looks rather fetching. That is one thin coat over the entire thing, just for some added protection. Then the real impact resistance treatment started. Anywhere most at risk of flying stones (and this took a lot of thought and planning!) got many more coats of Plasti-dip layerd up to build a thick layer. The front inner wings, footwell ends, fronts of the sills, chassis rails, fronts of the floorpans, front face of the crossmember, the rear wheel arches, the rear valance were the main areas for serious protection. When I have finished the metalwork mess that is the front wings, their undersides will get the same paint and Plasti-Dip treatment.
It's impossible to see, but there is about 1/8th inch of clear rubber over this lot. I am fairly happy this will give the impact resistance required, I would like more everywhere but literally cannot afford any more of the stuff. I shall see how it fares after hard use on a few road trips, and maybe apply more over the winter if required.
Oh, I will have to do the bits underneath the spit brackets later of course...!I am very pleased with how it has turned out. Very pretty and very solid, if you hit the panels they now don't really go clong. That's the Plasti-Dip taking up the impact!
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