75ZT Community

Mad-Monkey · Posted 17 Jan 2012, 10:51 #1

Been painting a couple of parts with heat paint that requires curing in an oven to reach optimum resilience. However the paint is re-flowing when hot and then setting like concrete when cool andim getting flat spots where the item is lying. How do you eliminate the flat spots?

It's only one type of paint as the other I used worked a charm, it just went yellow which according to google was the result of using a gas flame. Being primer I wasn't bothered.

The parts will be lacquered but I don't know how many sins you can hide with lacquer. :-)

Bermudan 75 · Posted 17 Jan 2012, 11:22 #2

Hi David,

Flat spots? is that where the part is resting on the oven tray? If so try hanging the item from one of the oven racks with some wire.

Cheers

Mike

Mad-Monkey · Posted 17 Jan 2012, 12:01 #3

Yes it's from the tray, sorry forgot to say. Will I not get marks from the wire as the paint softens under it though?

PaulT · Posted 17 Jan 2012, 12:13 #4

It has to be rested or suspended somehow so you have to decide which method will be the least detrimental

Jumper · Posted 17 Jan 2012, 14:10 #5

If the paint is flowing it sounds like it's too hot and reaching its flow viscosity level rather than the curing temp. That could also produce the 'yellow' effect (more likely to be beginning to scorch the paint by burning off the solvents). If it is too hot it might be because the oven is a domestic (?) one and not the same as a purpose-built kiln (air flow within the oven will be much more forceful - especially if fan-assisted). Again, if this is the underlying fault, the hot paint will 'flow' and build up at the lowest edge, so suspending the work piece will be critical! Solution: usually paint will cure at much lower temps than you might think, it's only to promote vector currents to draw air over the surface to aid evaporation of solvents in the paint. Does the paint container have instructions?

Mad-Monkey · Posted 17 Jan 2012, 17:17 #6

Thanks for that. The primer needed temps of 400c plus IIRC way higher than my oven so gas mark 8 and the primer was fine albeit yellow. The top coat tin just said increasing heat to cure so baked at varying temps over 30min, I take it a low temp then would suffice?

Jumper · Posted 17 Jan 2012, 22:05 #7

400 degrees? For primer? Wow, that seems as hot as hell for paint, I used to heat glass powder to that for enamelling purposes!
If top coat tin doesn't specify temps, then proceed from the low temp range gradually getting higher until the effective temp is reached. I hesitate to recommend exact temps as I have no idea what coating you are using - but I would not put any paint product in a domestic oven (especially a gas one!) at a temp in excess of, say, 20 degrees Cent. as you have to think about 'flash points'. I urge you to contact the manufacturer, or supplier, for a 'Hazards Sheet' - should be supplied with all chemical compound products. Think about it, if your car was in the body shop for paint work what would happen to it at those temps? Take extreme care!

Duncan · Posted 17 Jan 2012, 22:40 #8

I know it sounds daft, but was it 400C or 400F? A lot of these paints come from the good old US of A.

Jumper · Posted 18 Jan 2012, 14:53 #9

Yes, easily done! But that still equates to 204 degrees C. Sounds dangerous to me. Even with both my belt and braces on. If anyone knows different, please come on.

Mad-Monkey · Posted 18 Jan 2012, 15:08 #10

I'll dig out the can. Basically it was hotter than the domestic oven could handle and googling confirmed that some paints do need this heat, albeit perhaps not in a gas oven. The paint can handle temps up to 1600 degrees IIRC. The can came with a warning not to exceed the maximum temp of the part being painted and that was it. Once baked and flatted back with some 1200 wet & dry the finish was brilliant on the primer.

The black I'm using is only good to around 350 or thereabouts degrees C but when I bought it there was no mention of baking it. The instructions then said to bake it and it caused it to reflow. I've had to sand it back a bit and re-paint. Having to do two sides though is a pitta as it needs a good 24 hours to handle. I've done this and left it on a hot radiator to see if than helps stiffen the paint up. It marks so easily when painted and air dries. It was like concrete after baking, just had flat spots where it rested and looked rubbish!

Jumper · Posted 18 Jan 2012, 18:51 #11

Some coatings do need this heat (1600 deg), but invariably in a purpose-built kiln (potters or glass worker's type) that is non-fan-assisted to guarantee a stable atmosphere allowing predictable and controlled temps. I used to be a stained glass restorer - hence my interest.
The coatings are used for high performance components like ceramic brakes calipers on rally or supercars etc. (just thought, you probably know that!).
The reflow removes faults caused by amateur application (not saying that's you!) and gives a mirror finish for clear lacquer top.
The marking of the surface when air- dried is only because it hasn't been cured properly. It's called 'case hardening'.
Sorry if you know all this, all meant with best intentions! Good luck.

Mad-Monkey · Posted 19 Jan 2012, 09:09 #12

I don't know very much to be honest. I google what I think I need to then get annoyed when I realise I should of read more

I will have to see if I can balance the item on some pin points so I can touch up the marks and then clear coat them. Thankfully the clear coat does not need to be cured in an oven and still has a heat tolences of a couple hundred degrees C.

Mad-Monkey · Posted 19 Jan 2012, 09:19 #13

On the can of primer I've got muddled on here with my degrees C and F. I baked it at the right temperatures except the last as I couldn't go that hot, but didn't explain it too well.

The primer needed three curing temperature sessions. 200F then 400F and finally 600F each at 30min with 30min cooling inbetween. I got the primer finished quite well, albiet yellowed but then I wasn't too fussed as it was only primer. It's final heat resistance is 2000F not 1600C, got that bit wrong.

The black gloss I'm using is resistant to 300C and the can simply says "slowly heat then cool, this will finalise the curing process". No temps are mentioned.

So... Is reflow needed for the paint to cure properly and to get the professional smooth finish, or will it cure at lower temps without reflow. I used gas mark 1 to start with, which if its resistant to 300C it shouldn't reflow until I reached over 300C surely? Otherwise I'm going to have a painty mess when the item gets hot.

Jumper · Posted 19 Jan 2012, 12:41 #14

Well again, I don't know what particular coating you are using, or indeed what items you are painting. Suggest you contact supplier or manufacturer for precise recommendations, then there cannot be any doubt. If you take advice via tele-con, ask for an email from them to confirm, as (no disrespect to any individuals), not all staff are fully conversant. As for 'reflow', it is best to follow instructions exactly - whatever the process may be for.

Mick · Posted 19 Jan 2012, 13:35 #15

Just a thought............. A gas oven produces a lot of condensation, whereas an electric oven will be bone dry....... I'm no expert in the function of one as opposed to the other but it may be a factor.

Jumper · Posted 19 Jan 2012, 13:47 #16

Mick wrote:Just a thought............. A gas oven produces a lot of condensation, whereas an electric oven will be bone dry....... I'm no expert in the function of one as opposed to the other but it may be a factor.

Very true(ish)! Because the gas flame is burning oxygen it produces water vapour as a result (your point). That water can not condense because the internal oven temp is to high. It remains as vapour (steam) and forms a compound with the paint, giving a 'bloom'. I think! But I'm not a chemist!
Another thought. The colour coat will always 'cook' at a lower temp than the primer because then the primer does not have a chance to melt and merge with the top coat. Also, work pieces can be suspended by wire thro' 1/2 mm holes in it if they can be hidden upon assembly, but if the top coat has to reflow the paint might well sag and pool if surfaces are not horizontal.

Mad-Monkey · Posted 19 Jan 2012, 15:52 #17

Unfortunately there is no holes in these bits but I have an idea I might try. I've finished the last coat today cleaning up from the disaster earlier in the week so I'll let you know how it goes.

I've still got to lacquer it after so I could be doing it again yet lol

Bermudan 75 · Posted 19 Jan 2012, 17:03 #18

Is the oven you are using the same one to be used for your Sunday roast? :roll:

Mad-Monkey · Posted 19 Jan 2012, 21:07 #19

Yeah but sssshhh!!

75ZT Community

75ZT Community

Curing Paint in an Oven by Mad-Monkey