Paint for underbody repairs

[e833fef]

I've got an old Z3 I'm looking to try and tidy up a bit and sort some of the underbody corrosion, so sills and a few other bits. Looking at options for what paint to use to overcoat areas I've had to strip back, mainly for the first coat. Since this is just for touch-up work underneath anything sprayed is no good, I don't have the room to do this, and don't have a decent compressor or paint gun, so this will have to be brushed on. I'm also assuming my prep work won't be great just due to working in a confined space, so need something fairly tolerant of mediocre surface prep (i.e surface won't be keyed well, but all rust will be treated prior to prep). I do plan to try and do a decent job of it where possible, but want something tolerant just in case! I will say as well that I couldn't care less how good it looks when it's on there, the car's not a show piece up top, let alone underneath where nobodys looking, the main goal is to keep it solid.

So far I've ruled out epoxy primer due to how prep sensitive it appears to be, that leaves me with either:

High zinc primer (I.E Bilt Hamber Eletrox) - like the galvanic protection it offers but from what I've heard it's adhesion and mechanical strength isn't great, and would require overcoating with something else. Don't know if there are any formulations with high adhesion/mechanical strength or if they are all much of a muchness? That said I will have to use a bit of it on welded panels just to ensure good coverage inside cavities I can't get to afterwards...

Zinc Phosphate primer - like the sound of this as it claims to have good adhesion, I.E should be fairly tolerant of not great surface prep, assume like other primers it needs to be overcoated after the fact? Does it only stick well to bare metal surfaces or does it stick well to other paint, seam sealer, underbody coatings, etc, as well?

Epoxy mastics - from the sounds of it these are more tolerant of surface prep than the epoxy primers, and some seem to include zinc phosphate as well which I quite like the sound of, though don't know if these would adher as well as a "regular" zinc phosphate paint? Having had a quick look at the data sheets for jotamastic 87 and 90 they seem to require a minimal level of surface prep, is there any reason (aside from cost presumably) not to go with one of these over a standard zinc phosphate primer?

Any other options that I'm missing or are those 3 the best options for what I'm looking for? Also, what are people putting over these as a hard wearing topcoat nowadays? Not touching something like hammerite with a 6 foot pole, and tempted to avoid stonechip as well, unless it can be trusted when put over the top of a good primer (I.E it won't crack and let moisture in)? Appreciate a lot of this has been said before, but most of what I found when searching was either old, or was mainly about full restorations where 2k primer is the best option due to access, etc, which I don't have!

 

[steveo3002]

nothing wrong with epoxy , you make it sound like you dont want to prep it at all

 

[ajlelectronics]

So far I've ruled out epoxy primer due to how prep sensitive it appears to be.

Prep. is no different to lesser coatings. Clean and suitably abraded is all it requires, no coatings will adhere properly without.

 

[Wildefalcon]

Epoxy zinc phosphate?

 

[steveo3002]

Zinc Phosphate primer - like the sound of this as it claims to have good adhesion, I.E should be fairly tolerant of not great surface prep, assume like other primers it needs to be overcoated after the fact? Does it only stick well to bare metal surfaces or does it stick well to other paint, seam sealer, underbody coatings, etc, as well?

are you expect the rust saving properties to still work over old coatings and sealer etc? because it wont do jack , just spot repair any trouble areas if you refuse to strip it all bare , its like wearing a hat on your hat -no point at all

 

[bricol]

My experience under a 34 yr old Lancia?

Unless you blast the bottom, or somehow otherwise get similar roughness, anything epoxy will not stick was well as you hope - I've tried a few. I'm not sure you can get it with flap wheels etc.

If you are wire brushing, flap wheeling etc, any decent solvent primer will stick pretty well - paint it on, overlap over edges of underseal, seam sealer. Wait a day or two, cover with any other top coat, preferably of different colour so you can see it over the primer, hopefully not black either - I used, and still use, machine enamel that was being chucked from work, simply because I had it, not because of any thoughts of it being the best thing for the job. Wait until dry, apply chosen underbody coating - I've used waxoyl underseal, 3M body shultz - as it's black, you can easily see where you've slathered it over the paint - and later on, see any damage from stone chips easier. Easier to apply with a shultz gun and compressor - doesn't need to be a large one.

Conventional paint over a blasted surface sticks well too of course.

After ten years of daily use I found that the epoxy over blasted surfaces needed blasting or serious flap wheeling to get off.

However, epoxy over non-blasted, especially smooth, un-touched, non-rusty (originally!) metal had either already detached or was easily poked off with a screwdriver etc, causing issue with water now hidden behind it, not previously suspected as the external surface still looked a good, continuous layer - it was, just not actually attached to the underside!

Normal paint was still stuck, and protecting, to such surfaces pretty well - needed proper scraping, sanding off. Of course areas were failing, but were noticeable. And the paint around such areas was still stuck.

For more serious rust, not yet gone through, I found a small spot blaster cleaned it up well to give me more confidence what I painted over it would stay there.

I use an underbody coating over the paint to give it some protection from flying crap - more bounces off than penetrates it. If you use a shultz gun when under a car on ramps - wear old clothes, and a cheap full face mask/screen - easier to chuck the screen than get the underseal spots off your face, out of your hair!

34 yr old Lancia in the car park at work right now gives me some degree of confidence I've done something right somewhere. I had a good look under it again at the weekend - all still looks good.

Just to add, that while I've used the zinc rich primers, I'm not convinced they do anything more than a conventional primer once cured - I know the theory, I'm just not convinced the zinc links up enough to do what they claim.

 

[ajlelectronics]

Unless you blast the bottom, or somehow otherwise get similar roughness, anything epoxy will not stick was well as you hope

Which is contrary to the evidence of everyone else's experience. Check the TDS for the chosen paint and that is the correct information on preparation. Lechler 29107 for instance is clean and abraded substrate, between 120 and 180 grit. I have never had a failure, except the Middlebridge that I asked someone else to paint. That failure was due to not bothering to clean it first.

I have less experience of epoxy mastics, but popular opinion suggests similar success rates.

NOTHING, will adhere to oily and smooth substrates.

 

[steveo3002]

my findings are jotun sticks well with diy prep , i do the best i can but its far from perfect

and yes any paint will stick better to sandblasted steel

edit** over my 35 years ive tried most other rust primers and miracle jollops , epoxy still comes out leagues ahead of the others ive tried , plus it chemically hardens in xx hours so you can crack on with the next steps quickly

 

[barracuda816]

Another +1 for epoxy or epoxy mastic. I like also that is is chemically resistant to things like petrol and brake fluid. I have had leaks that washed off solvent primer and it makes a real mess of most single component shutz type stuff.

My advice is put a little more thought into the prep. Nothing will make up for poor prep. You don’t need to sandblast everything but do your best to get rid of rust and scuff everything up with 180. After a really good cleaning, wire brushing and a good scuffing up a box of scotch pads will be a godsend to help scuffing up. Pay attention to the edges and corners.

 

[bricol]

Which is contrary to the evidence of everyone else's experience. Check the TDS for the chosen paint and that is the correct information on preparation. Lechler 29107 for instance is clean and abraded substrate, between 120 and 180 grit. I have never had a failure, except the Middlebridge that I asked someone else to paint. That failure was due to not bothering to clean it first.

I have less experience of epoxy mastics, but popular opinion suggests similar success rates.

NOTHING, will adhere to oily and smooth substrates.

I can only offer my experience after 21 years of owning the same car, using it every day for a lot of that, it never being garaged, never being particularly looked after - get in, drive. Used all year round, having applied coatings to the underside when I got it after some remedial work, and doing more work 7 or 8 years ago.

Blasted surfaces - epoxy sticks like sticky sh it to a blanket - and is still there 20 yrs later. Non-blasted - it's not. Had failed, not everywhere admittedly, but enough for me to never use it on non-blasted surfaces. But on similar non-blasted, a lot of conventional paints are still stuck.

 

[steveo3002]

another welcome bonus is that epoxy will sit in the shed for a few years after being opened and still work vs airdrying often starts to go off in no time

 

[Wildefalcon]

If you are determined to avoid proper preparation then at least rinse it off with brake cleaner and scrub it with detergent and wire wool.

 

[e833fef]

nothing wrong with epoxy , you make it sound like you dont want to prep it at all

not at all, but I know that things rarely turn out perfect, so don't want to have to redo it just because one difficult to reach area wasn't perfectly scuffed up. That said, I think rust and oil free go without saying, I'm not expecting it to work miracles

are you expect the rust saving properties to still work over old coatings and sealer etc?

why would I expect that? I'll have to put a coat over seam sealer in places and up to the edge of the area I've stripped for repairs so want to know if it's going to have any issues sticking to those areas. Spot repairs are all I'm doing, not going to waste paint or time painting over underseal except at the edges of repair areas.

Unless you blast the bottom, or somehow otherwise get similar roughness, anything epoxy will not stick was well as you hope

assume that means epoxy mastics as well as primers? any info on the brand you've tried, and was it smooth (i.e flapwheeled) steel or abraded in any way?

my findings are jotun sticks well with diy prep , i do the best i can but its far from perfect

good to know, which one did you use? seem to be a few that claim more or less the same things. I was leaning towards something like jotamastic 90 or 90a if I go that route.

 

[bricol]

assume that means epoxy mastics as well as primers? any info on the brand you've tried, and was it smooth (i.e flapwheeled) steel or abraded in any way?

I used the rust busters epoxy mastic the first time, 20 plus years ago. On both flapwheeled metal (40/80 grit - so definitely marked, not perfectly smooth) and blasted metal. Some was spot blasted by me, some, like the rear inner wheel arches and the rear subframe, I paid a sand blasting company to come and do it - covered the drive, garden, most of the road in sand. Took longer to set up and pack away than to do the job.

21 years later, the rear inner arches and subframe are pretty much perfect - no signs of failing of the coating. The rear subframe has a thin line of orange on one small length (an inch or so) of seam on one side).

Areas underneath where I spot blasted, where panels met, vertical bits poked down etc, all seem to be pretty much as they were. Even the rear valances behind the wheels, while the underseal gets blasted off by road grit over a few years, the paint looks intact.

A fair section of the flap wheeled area - the front cross member comes to mind - had large areas, a couple of square inches in area at a time, totally detached from the metal - looked good, but was only hanging on by being attached to the surrounding epoxy still being attached - could be pulled away and broken off by finger nails. And of course water got under it . . .

In my opinion, nothing to do with the epoxy mastic - just the surface roughness of the metal - it simply didn't have enough of a key to keep it attached properly.

I've since used jotamastic on other areas, and on other cars - I only apply it blasted areas - 7 years later it still looks good.

With both, when the epoxy has cured, I give it a coat of another paint, then underseal is it's underneath being blasted by road crap.

I'll happily use epoxy - but only on areas I am confident I can give a suitable finish. Everywhere else, I prefer to use a conventional primer, a layer of two of paint over it, then underseal.

I power washed the underside of the Lancia last weekend to remove the winters worth of road salt, dirt etc - a large amount of crap as I used back roads home on an evening. Put it over my pit the day after to give it a good coat of looking at - I want to catch any damage to the coatings before the metal gets damaged. Nothing I see makes me consider changing what works for me.