Bare metal underside prep

[Syv]

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.

Any recommendations on solvent based degreasers? I was initially planning to use water based one (like Bilt-Hamber Surfex HD), but now have serious doubts.

What's the pro tip on cleaning before primer? Solvent based degreaser, than mineral spirits and good to go?

 

[bert]

I use trident at work both the slow and fast versions although at the moment the slow is almost like using fast and the fast evaporates on the cloth

 

[Burdekin]

Any recommendations on solvent based degreasers? I was initially planning to use water based one (like Bilt-Hamber Surfex HD), but now have serious doubts.

What's the pro tip on cleaning before primer? Solvent based degreaser, than mineral spirits and good to go?

I use Upol as I can buy it from my local Euro car parts shop. I also use a 50:50 methylated spirits and water mix as my final clean before a tack cloth. I'm no pro though so it's just how I do things. Painting is one of those things, plenty to go wrong so when you find a method that works stick with it.

 

[Syv]

getting ready to spray jotun, any recommendations on spraying underside? The car is on jack stands.

Should I stick with regular 2.5 tip gun or invest in something like swivel cup spray gun?

 

[Dcal]

2.5 tip works well.
If you can get a swivel cup gravity gun with a 2.5 fluid tip that would be even better.
I know a pressure pot would certainly work, because that's what I used, if you can get access to one.
The 3m PPS system or similar might also work but I havent tried them.
Spraying above your head is not easy with a standard gravity gun.

 

[Burdekin]

I bought a wee cheap SIP pressure pot touch up gun might help in tight areas for you. It actually sprays really well and think it was about £20. Think I got it from Halfords a few years back. I'd recommend painting all the difficult areas first with a brush.

 

[Syv]

Thanks for the comments!
You guys thinking about those (1, 2 pic)?
I was more prone to the swivel type gravity gun (third pic) for major work, and small brush for hard to reach areas. Now have to rethink it, cup on top will likely to limit movement... Any drawbacks on using pressure pot guns? 380L/min compressor should be efficient enough?

 

[Dcal]

Pressure pots don't really use any more air. The air cap will make a bigger diffrence.
They are a pig to clean out so are not perfect by any means, but great for a job like yours. You might even get away with a smaller fluid tip.

I think, of the three, the bottom gun is the best option, if you can get a big enough fluid tip setup.
Looks interesting, have you a link to it?

 

[Syv]

Pressure pots don't really use any more air. The air cap will make a bigger diffrence.
They are a pig to clean out so are not perfect by any means, but great for a job like yours. You might even get away with a smaller fluid tip.

I think, of the three, the bottom gun is the best option, if you can get a big enough fluid tip setup.
Looks interesting, have you a link to it?

yeah, sadly swivel cup spray guns max out at 1.8 tip, haven't found any bigger ones. I'll probably stick with the suction type and 2.5 tip. Sealey makes one, like in the picture (Recommended Air Pressure: 57psi, Air Consumption: 3-4cfm)

 

[Burdekin]

I'm back using Jotamastic 80 and it lays flatter with a 1.8mm cap. If the swivel gun isn't too expensive I'd give it a go, maybe need to thin it a bit more or warm it up and a bit more pressure. Good time of year with the warmer weather so 1.8mm should be ok.

 

[bert]

I prime with a 1.8mm tip and can lay on a stupid amount of primer in one go so I don't really think a 2.5mm tip is needed. You certainly wouldn't be able to use it for anything other than high build primers

 

[Syv]

did the order of jotun jotamastic 80 and thinner 17, also got the swivel cup 1.8 spray gun. Last things: seam sealer.

I did the research and it was discussed before to use seam sealer between coats of epoxy mastic.
The big question is what good seam sealers would you recommend? I can probably start looking for seam sealer gun, ideally would like to replicate oem look. Brushable maybe? Tiger seal, 3M, Dynax seam others maybe?

 

[Dcal]

Any polyurethaine sealer will work very well.
Not sure what the OEM look is for your car, but I don't get on with brushing seam sealer and if I'm after a brush finish I gun on the polyurethane and run the brush wetted with some thinners over that.
To be fair, under a car I'd be more interested in getting it well sealed than getting an OEM look.

Here's one I did earlier.

25 year old Alfa 75 back from the brink (hopefully)

Thanks @Exuptoy Nearly there with this bit but every time I think I'm finished a I find something else! But it's good to finally say enough is enough and get some paint on. Any job is better than getting underseal off, a bit of welding is quite therapeutic.

www.mig-welding.co.uk

 

[Burdekin]

did the order of jotun jotamastic 80 and thinner 17, also got the swivel cup 1.8 spray gun. Last things: seam sealer.

I did the research and it was discussed before to use seam sealer between coats of epoxy mastic.
The big question is what good seam sealers would you recommend? I can probably start looking for seam sealer gun, ideally would like to replicate oem look. Brushable maybe?

I use Sikaflex 221. Did the car have seam sealer on the chassis before. Places like the engine bay I mask up apply, level it out and just go over with a stiff paint brush to replicate factory. I personally wouldn't apply it thick on the underside as it's more likely to fail I think. Just a small bead and use your finger or a tool to push into the seam and leave it so you can hardly see it but it's sealing the join is how I prefer to do it and in my head less chance of it lifting over time and failing.

 

[Dcal]

I personally wouldn't apply it thick on the underside as it's more likely to fail I think. Just a small bead and use your finger or a tool to push into the seam and leave it so you can hardly see it but it's sealing the join is how I prefer to do it and in my head less chance of it lifting over time and failing.

Not often (if ever) I'd disagree with you @Burdekin but I would, slightly, on your last point.

I agree too much sealer is bad, but I don't think the smoothing with your finger is best practice.
I know we are all guilty of doing it, myself included.

You need the sealer to have reasonable thickness so it can move, or it can crack or tear.
This isn't much of a problem on solid surfaces but can be on a joint that can move.

For bits where there is a lap joint (like where a chassis meets the floor) a nice bead of seam sealer twice as wide as it is deep is good. You don't need or want a huge bead, look at how pretty much all door skins are sealed to the frame.
It's not easy to do well (like on a OEM door skin) but the underside is a great place to practice.

I'm that sad, I think a perfect bead of seam sealer is a thing of beauty, but maybe that's because I know how hard it is to get right.

 

[Burdekin]

Not often (if ever) I'd disagree with you @Burdekin but I would, slightly, on your last point.

I agree too much sealer is bad, but I don't think the smoothing with your finger is best practice.
I know we are all guilty of doing it, myself included.

You need the sealer to have reasonable thickness so it can move, or it can crack or tear.
This isn't much of a problem on solid surfaces but can be on a joint that can move.

For bits where there is a lap joint (like where a chassis meets the floor) a nice bead of seam sealer twice as wide as it is deep is good. You don't need or want a huge bead, look at how pretty much all door skins are sealed to the frame.
It's not easy to do well (like on a OEM door skin) but the underside is a great place to practice.

I'm that sad, I think a perfect bead of seam sealer is a thing of beauty, but maybe that's because I know how hard it is to get right.

Fair enough. None of my cars had seam sealer on the bottom of the floor from factory and never really found open seams to be much of a issue. I see some folk seam seal every seam under a car which for me is overkill, not needed and a added layer that may fail over time. I personally only seam seal where there are gaps to stop moisture getting in or muck could cling to otherwise I rely on the epoxy primer, stonechip and topcoat. If there is a open gap I fill the gap with PU adhesive and this then gets over coated. Every car has it’s problem areas where muck can collect, important to consider those areas carefully, fill them properly or keep them clear so they can be washed out. As ever there’s more than one way to skin that cat.

 

[bluepurple]

Really interesting reading. I am in the process of a similar restoration on a Honda S2000, on my back under the car on jackstands.

Do you have a link to the swivel gun you used for the Jotamastic?

 

[charlysays]

Any recommendations on solvent based degreasers? I was initially planning to use water based one (like Bilt-Hamber Surfex HD), but now have serious doubts.

What's the pro tip on cleaning before primer? Solvent based degreaser, than mineral spirits and good to go?

Use surfex HD at the outset before any prep happens. I did the subframe area on my car twice times with neat surfex, brush and a hose pipe. Diff had been leaking a bit so there was some gunge to shift but thankfully no waxoyl or shutz.
I used 99.9% IPA as a final degrease after completing prep, mopped up with paper towel and microfibre cloths. I often use brake cleaner but found it can attack OE seam sealer which can then contaminate metal with an invisible film of stuff which can cause fish eyes, IPA seems safe in that respect.