Bare metal underside prep

[Syv]

Hello folks,

I have a few questions about bare metal preparation. Underside cleaned with flap wheels/wheelbrushes, some rust spots treated with BH deox gel, some spots sand blasted. Not sure whats next, I was considering these options, but would like to get a pro opinion.

Option A:
BH Electrox zinc rich primer
BH Epoxy Mastic
Upol raptor or gravitex in some areas

Option B:
Jotun epoxy primer (barrier 90?)
Jotamastic 87 mastic
Raptor or gravitex in some areas

I'm aware that zinc primers require solid preparation for adhesion, so not sure if my work will count as good? I'll add some pics for judgement.

Also I want to get a nice finish from epoxy mastic, the idea is to spray it. Which of these (or others) products have smoothest finish? Maybe some of you will have first hand experience spraying epoxy mastics.

Cheers!

 

[brightspark]

if it was mine id do it in epoxy 2k primer 2 k topcoat then an underseal

 

[Wildefalcon]

I think your work is too shiny, no key, for the zinc epoxies. But that's a massively uninformed opinion.

 

[brightspark]

I think your work is too shiny, no key, for the zinc epoxies. But that's a massively uninformed opinion.

it would be real nice key a quick dusting over sand blasting or fine grit

 

[Burdekin]

You want the epoxy mastic straight onto the steel, you don't need epoxy then epoxy mastic. Epoxy mastics are designed to be more surface tolerant than straight epoxy primers so are ideal for what you are doing. They should all spray flat as long as you don't go too big with your nozzle, viscosity correct and enough pressure.

As above if you media blast it then you can expect a much better bond to the metal. International paints say a media blasted steel compared to a hand prepped one with a wire brush is up to 5 times better.

The non-woven discs will get you a decent key. Thoroughly clean with water based degreaser and then solvent based degreaser before spraying, I like the Upol stuff but will be other good brands. Keep cleaning until nothing comes off on the rag.

What I did was Jotamastic 80, raptor and 2k topcoat. I also wouldn't bother with the tinted raptor, my personal opinion was you need to lay down more to get the colour coverage and you're just better off using the non-tinted standard and then a couple of coats of topcoat all over including areas you were planning on not coating with stonechip.

 

[bricol]

I think your work is too shiny, no key, for the zinc epoxies. But that's a massively uninformed opinion.

Sounds like an absolutely correct opinion to me - based on my past experience of applying epoxy paints to wire wheeled, flap disced and blasted metal under cars.

 

[Dcal]

Welcome to the forum
What is it?

As above, it looks too shiny, but great work.
.If you go over it with coarse paper, say 80 or 100 grit, to a matte finish you will get far better adhesion.
As @brightspark said blasting would be excellent and quick but a good going over with one of the wee 2 inch da sanders, would be pretty dam good as well.
As @Burdekin said cleaning is super important as well.

I've used both the Jotun products and for this application I would just use 87A (or 90A) and forget about the zinc rich.
It sprays and dries far better than the Barrier.
Over thin the Jotun with thinner 17, use a 2.5mm fluid tip, apply two or three coats and let it dry overnight between coats and you will have an excellent base for the next coats.
You can apply the next coats after the Jotun dries overnight.

 

[Syv]

Cheers, it's 1998 honda integra type R.

Jotun mastic coverage looks really nice, but what about rust? I bet those pitted areas will still have some rust in it..

Maybe I should consider spraying electrox in those spots, grind it down and than full layer of jotun 87? I've contacted bilt hamber, they said electrox does not interfere with other primers.

Or this is just waste of time?

 

[Roger440]

Sounds like an absolutely correct opinion to me - based on my past experience of applying epoxy paints to wire wheeled, flap disced and blasted metal under cars.

+1

Dont think epoxy mastic will stick that well based on past experience. Maybe use an etch primer first which will bite on that surface.

 

[ajlelectronics]

+1

Dont think epoxy mastic will stick that well based on past experience. Maybe use an etch primer first which will bite on that surface.

Then your prep. was insufficient. The reason that epoxies have taken over is because their properties are far superior to all the conventional products. Etch primer in particular is rendered obsolete by epoxy; aluminium, fibreglass and plastics are difficult substrates all of which are handled easily by epoxy paints.

 

[norsa]

As AJ. I was late coming to the party but 2k epoxy is the way forward.
Nick

 

[Roger440]

Then your prep. was insufficient. The reason that epoxies have taken over is because their properties are far superior to all the conventional products. Etch primer in particular is rendered obsolete by epoxy; aluminium, fibreglass and plastics are difficult substrates all of which are handled easily by epoxy paints.

Agreed.

But very difficult to get something like this roughed up enough without resorting to blasting.

 

[bert]

I put 2k epoxy over 400grit prep so would put that straight on. Maybe a rust converter in the pitting before if you are worried.

Light sand then Raptor or mipa protect then 3 coats of top coat

 

[Dcal]

There seems to be two schools of thought here.
First is - the rougher the surface prep the better and the second it is - ok as it is.
I'm certainly of the first school.

I also prefer Epoxy Mastic where they could be remaining rust, because it is designed for this very application.
The chemistry is similar but the dry film thickness is higher, which makes a huge difference to the life of the system.

Epoxy mastic (which Jotamastic is just one flavour) is designed to be applied over well adhered clean rust.
The more rust you can remove the better but only to a point.

Sound rust will provide a better, grippier surface and so a longer lasting paint system, than the same surface that has been polished by the action of a high speed wire brush, or even a nice rust free surface like mill scale.
In fact it is much better to let mill scale be removed by the action of rust if you are not able to remove it by blasting, or some other mechanical means or by chemical dip.
The mill scale will let go from the surface of the steel before the primer from the mill scale, so you have a dodgy layer if you don’t remove it. This isn’t an issue in your case but worth noting for structural steel work.

Epoxy is a great glue and will stick to most things but if the “thing” isn’t well stuck to the layer below the system will fail eventually.
Good prep gives the epoxy another mechanism to increase adhesion, this works in two ways -
1. - It provides peeks and troughs that the primer can fill and mechanical lock the paint to the metal.
2. - It increases the surface area of the metal that the primer has to cover and all things being equal the larger the surface area the stronger the bond.

In my opinion the range of prep from excellent to crap is -

1. - Clean, pressure wash, (or some other cleaning method to remove salts and other contaminants) grit blast to SA2.5 (or SA3 if you have nothing better to do) check salt levels, surface roughness and cleanliness. If all is to spec, on with the primer (usually within a 4 hour window) If salt levels or some other issue is found, resolve the issue before (probably) blasting again, retesting and applying the primer.

2. - Clean, pressure wash, and mechanical prep to ST3 which is “very thorough mechanical preparation”. There is a bit of an issue here, because SA3 doesn’t have a surface roughness (while blasting does) but the "best" final mechanical prep, is a Bristle Blast finish. Then check surface is as “1” and on with the primer if OK.
A Bristle Blaster is a tool the closely replicates a blasted finish. They are expensive, can be slow, but do a great job and can sometimes be used where blasting can’t.

3. - Clean, remove all coatings, then mechanical prep to ST3 and sand to matte finish to provide a key in the metal. I would suggest 80 or 100 grit on bare steel. Clean again with a solvent wash and on with the primer before any flash rusting or contamination occurs.

4. - Clean, remove all loose coatings, then mechanical prep to ST3 and sand to a matte finish to provide a key. I would suggest 80 or 100 grit on bare metal and 180 or 220 grit on any remaining existing sound coatings. Clean again and on with the primer before any flash rusting of contamination occurs.

5. - Clean, mechanical prep to remove all coatings and apply primer without any further steps. This is where I think you are and it looks a dam good, but (relatively) very little extra work would really improve the surface i.e. get it to 4. I know the work you have put in so far so I’m not making light of it but as you have gone this far I’d certainly go the extra bit.

6. – Clean, quick rub down and on with the epoxy

7. - Quick rub down and on with the epoxy - If you go with this the results will be difficult to predict because it depends what contamination is present, but no paint I know of will stick to oil or dirt

8. - Paint the surface as is (see 7 above.)


1 or 2 above is overkill for a car in my opinion but it doesn’t stop me doing it. (without the salt and surface roughness testing)
Anything to 4 is really good
From 5 down the results might be OK but will be very unpredictable.

 

[bricol]

Then your prep. was insufficient.

I read that as his point. Putting epoxy on a wire wheeled, nice smooth surface isn't as effective as a proper prepared surface - as fully detailed above.

Certainly my own experience of ten years of use all year round underneath a daily driver Lancia - epoxy on blasted surface is better than conventional paint on more usual flap wheeled surface is better than paint on wire wheeled is better than epoxy on anything other than blasted surface.

The epoxy cures well on wirewheeled surfaces, but subsequently detached from the surface and let damp in . . .

 

[Ross365]

I'm a big fan of zinc phosphate primers; have used nothing else for the last decade. The OP refers to a "zinc rich primer" which I assume contains zinc metal rather than zinc phosphate. My personal opinion is that zinc rich primers are very good with the perhaps the question of how well they "apply to the metal"- I think they can be a bit viscous/stodgy. Many paints have that tendency, and yes I concede that they may produce a thicker coat, but how well is it bonded? My zinc phosphate just "jumps" on to the metal (well, it transfers for the brush to the metal effortlessly ) and finds its way down in to seams very well. But, you ask 10 people about such matters, and you get 26 different opinions!

 

[Syv]

Cheers for the lessons lads, looks like jotamastic will be the choise. I took some 80 and 120 grit samples, lights in the garage may add same flash but it does look keyd properly now. Does it look like it should now? Should I stick with 80 or 120 discs?

 

[Burdekin]

Nice, that looks spot on. I just sprayed my bare metal shell on the weekend, it started to flash rust after the water based degreaser so I'd skip that and just go with a solvent based degreaser.

Costco do bags of 36 microfiber towels which are ideal for pre-cleaning before primer.

 

[vw1]

Myself I stick with 80 grit key.

 

[Dcal]

+1 for the 80 grit