Paint mixing ratio question (specifically Jotun 90A WG)

[charlysays]

Got some painting to do tomorrow and noticed it has an annoying 3.5:1 mix ratio.

I have some colad cups which have 7:1 ratio plus a percentage column.
Now, 7:1 plus 10% looks very very close to the required 7:2 (3.5:1) ratio.

Could anyone familiar with mixing cups and maths confirm if it's possible to get damn close to a 7:2 mix using the 7:1 column on a mixing cup and an extra percentage of hardener? My cups go up in increments of 5% up to 30% but it is possible to bisect to get finer increments.

I can only make one batch size using the milliliter scale otherwise (175ml of paint plus 50ml of hardener, total 225ml)... Or I'll neck a can of baked beans for breakfast and use a ruler to work out the ratio in the empty tin and force dry the paint with methane lol.

 

[charlysays]

So to answer my own question, 7:1 plus 10% worked fine. I managed to do some calculations and if it were going to be an exact 7:2 ratio you want 7:1 plus 11%. 10% is close enough, epoxy cured fine
So for anyone concerned about the 3.5:1 mix ratio for jotun 90, get yourself some Colad mixing cups and use the 7:1 mixing ratio with an extra 10% of hardener added on the column which would normally be used for thinner. Sorted!

 

[123hotchef]

I weigh it now then I can mix it in whatever I like rather than the plasctis measuring cups

 

[charlysays]

I weigh it now then I can mix it in whatever I like rather than the plasctis measuring cups

Yeah I always weigh out laminating epoxy resin. Epoxy paints seem to be formulated to be mixed by volume though so doing it by weight isn't straightforward unfortunately. Every single different type of jotun has a different weight ratio/ density and the only people who publish these are SML paints and jotun 90WG isn't on their table. I could get the info off jotun I guess. The good thing with epoxy mastic is if you just let the residue cure in the mixing cup it peels out easily after a few days. Trying to clean up the cups in gun wash scratches them and rubs the writing off them making them useless after a few uses. I might email jotun for the density info for future use. I have loads left so I'll be using it for a few years I imagine.

 

[123hotchef]

Yeah I always weigh out laminating epoxy resin. Epoxy paints seem to be formulated to be mixed by volume though so doing it by weight isn't straightforward unfortunately. Every single different type of jotun has a different weight ratio/ density and the only people who publish these are SML paints and jotun 90WG isn't on their table. I could get the info off jotun I guess. The good thing with epoxy mastic is if you just let the residue cure in the mixing cup it peels out easily after a few days. Trying to clean up the cups in gun wash scratches them and rubs the writing off them making them useless after a few uses. I might email jotun for the density info for future use. I have loads left so I'll be using it for a few years I imagine.

I found the weight mixing ratios chars online. I just printed it off for futre refrence

 

[123hotchef]

Here we go

 

[charlysays]

Here we go

View attachment 336720

It's all good providing you aren't using jotamastic 90 WG. There's no weight for the comp B for jotun 90 WG. As we can see the WG and Standard comp B for 87 are different weights. More info needed from jotun themselves I think although the difference is probably insignificant between the WG and Std comp B for jotun 90. I read on a marine forum people finding errors in this table too so probably best to do your own calcs to double check... but if it works, it works

 

[GRW]

Excuse my ignorance... but what is Jotun please, and what sort of thing would you paint with it?

 

[charlysays]

Excuse my ignorance... but what is Jotun please, and what sort of thing would you paint with it?

Its an industrial grade epoxy mastic (paint). Its primarily aimed at repair and maintenance operations on boats (inc below the waterline), offshore steel structures, bridges etc. You can paint a variety of substrates with it including aluminium but not sure what else.
It's surface tolerant meaning you can paint over visible corrosion (but not loose rust) and it will encapsulate it and stop it progressing and the winter grade version will cure at temps down to -5C. It seems to stick like s*** to a blanket too. I painted it over some areas of degreased and abraded OE underseal and I was picking holes in the underseal rather than picking the product off. 15 year life expectancy in marine environments means it can last a very long time applied to the underside of cars if proper prep is carried out. Lots using it as a standalone one stop coating under cars and it holds up well. Quite cheap per litre too and isocyanate free meaning just normal gloves, filter mask etc is fine.

 

[GRW]

Its an industrial grade epoxy mastic (paint). Its primarily aimed at repair and maintenance operations on boats (inc below the waterline), offshore steel structures, bridges etc.
It's surface tolerant meaning you can paint over visible corrosion (but not loose rust) and it will encapsulate it and stop it progressing and the winter grade version will cure at temps down to -5C. It seems to stick like s*** to a blanket too. I painted it over some areas of degreased and abraded OE underseal and I was picking holes in the underseal rather than picking the product off. 15 year life expectancy in marine environments means it can last a very long time applied to the underside of cars if proper prep is carried out. Lots using it as a standalone one stop coating under cars and it holds up well. Quite cheap per litre too and isocyanate free meaning just normal gloves, filter mask etc is fine.

Thanks for that. Sounds like good stuff!
I know nothing about painting and paint processes, but understand that some paints need specialist equipment and a deep understanding of how to use it.
I need something eejit proof that you just mix with some thinners, stir put in a cheapo gun and have at it ..... but if its easy to apply and for underbody it sounds ideal!

 

[123hotchef]

I have to 90AL to do my shogun chassis, used the 87 on a few extras I made from my off roader too. It stands up well.

 

[charlysays]

Thanks for that. Sounds like good stuff!
I know nothing about painting and paint processes, but understand that some paints need specialist equipment and a deep understanding of how to use it.
I need something eejit proof that you just mix with some thinners, stir put in a cheapo gun and have at it ..... but if its easy to apply and for underbody it sounds ideal!

I think it's about the best thing out there tbh. The only time I wouldn't use it now is on an old banger which has had oil based/ greasy coatings previously applied. Without a major stripdown and thorough cleanup job there would be too much risk of grease contamination to use any kind of paint. Sometimes you can partition an area off. I used to use rust converters, re-abrade, bilt hamber electrox, then gravitex and it worked well but wasn't as effective I don't think or as surface tolerant.

ST2 surface prep is the minimum which Jotamastic needs which is "thorough powertool cleaning which leaves a rough substrate with a faint metallic sheen".... so in other words it is easy to use due to this. Just a needle gun with freshly ground needles used at an angle to the substrate will achieve st2. I always aim for st3, which for me involves using straight bristle low rpm wire wheels then abrading with 40-80g. If you can media blast it that is the ideal prep.

yes it needs two parts mixing but that only adds a tiny amount of time to the overall job and if you use jotamastic as the sole primer and topcoat underneath that saves a load of time versus applying several layers of different coatings.

It's best brushed on to underbody areas so that it gets worked into seams and craters better.

For top sides lechler 29107 is best. I think it's also surface tolerant to a degree too. This forum has made me epoxy mad hahaha.

 

[charlysays]

I have to 90AL to do my shogun chassis, used the 87 on a few extras I made from my off roader too. It stands up well.

I went for the 90A WG too. It claims better barrier effect and less cathodic debonding (fancy term for rust blisters?). In theory, if it chips, the alu will protect the substrate even at the breach/ chip which buys you time and makes coating repairs smaller, much like galv does but obviously less efficient.

 

[fizzy]

Leighs paints are even better. Made in the NE - sadly taken over by Sherwin Williams but still made in the same factory in the UK.
Not cheap at all - aluminium in it for corrosion resistance and glass flakes for abrasion.

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Promain Paints: The UK’s largest commercial / industrial paint and surface coatings company with over 35 years experience in industrial paint supplies.

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[charlysays]

Leighs paints are even better. Made in the NE - sadly taken over by Sherwin Williams but still made in the same factory in the UK.
Not cheap at all - aluminium in it for corrosion resistance and glass flakes for abrasion.

#1 Industrial Paint Suppliers & Distributors in the UK

Promain Paints: The UK’s largest commercial / industrial paint and surface coatings company with over 35 years experience in industrial paint supplies.

www.promain.co.uk

I would use this if blasting for sure. Looks excellent.
Jotun is more surface tolerant tho (st2) than this by the look of it, says minimum prep is st3 which can be hard to fully achieve underneath cars without blasting .

 

[123hotchef]

I went for the 90A WG too. It claims better barrier effect and less cathodic debonding (fancy term for rust blisters?). In theory, if it chips, the alu will protect the substrate even at the breach/ chip which buys you time and makes coating repairs smaller, much like galv does but obviously less efficient.

I didnt go for the WG this time I did with the 87 though

 

[charlysays]

I didnt go for the WG this time I did with the 87 though

I think the WG is ideal for Brits working in sheds over the winter... You need to heat the substrate a little before applying and before subsequent coats to ensure its fully dry but that's it... It'll still cure in all but the coldest UK weather. Where I am in West Wales -5 is basically unheard of. Takes its time though...3 months to full cure and about 3 weeks before it's ready for service.

 

[GRW]

The only time I wouldn't use it now is on an old banger which has had oil based/ greasy coatings previously applied.

Yep, this sums up what I've got pretty much
(I'm in West Wales also. It might not get that cold according to the Mercury... but it more than makes up for that in damp, more damp, with a generous helping of wet & windy - and that's just the summer

 

[fizzy]

Yep, this sums up what I've got pretty much
(I'm in West Wales also. It might not get that cold according to the Mercury... but it more than makes up for that in damp, more damp, with a generous helping of wet & windy - and that's just the summer

Humidity is the biggest problem. We often get over 90%.

 

[charlysays]

Humidity is the biggest problem. We often get over 90%.

I think with surface heating that's not a major issue though? I may be missing something but if the substrate is warmed with dry heat to well above ambient for a while it is dry and can accept paint. I still try to paint when humidity is below 90% though. Anything above and there's often a film on everything.
Underneath a car on a lift it takes surprisingly little heat to get the substrate up to 20c though, 1kw of electric patio heater for example. All the heat goes up where it's needed.