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Why it is bad to mix match hardeners with paints and why sometimes it could be good?

Hello Everyone, with most of us sat at home (myself included) I thought I would go into an in depth explanation of 2K hardeners and why it is bad (or good) to mix match brand hardeners.

One of the main things I came across in my past job was visiting bodyshops who had paint issues. One of them being that the 2K or 2 pack direct gloss hasn't cured properly. This usually goes one of two ways, they have used the in-correct hardener and admit that they should have used the correct one or i get the typical phrase "Listen, I've been painting 25 years don't you tell me what to do I've used U-pol hardener with all other Direct gloss paints and it works fine". They are the nightmare customers!

I'm going to go a bit technical into how the paints cure but not too technical you think i'm trying to look clever or a know it all! If you know me you know that's not me anyway.

So firstly how do paints cure? Well most of you already know that it is the Isocyanate in the hardener which is correct but what about the other part of the product? the Clear or the Direct gloss?

To get a correct cure two things need to be matching. Lets discuss a clear coat.

We have the clear coat which has (OH Groups) and the hardener which has (NCO Groups) now these groups have to match each other to get a correct cure. Before we get any smart comments yes they don't need to be matching 1:1 it can be slightly different but we're not going that in-depth.

Clearcoat: The clearcoat contains OH groups this is measured on the solid content of the resin in the clearcoat. This OH group needs to match the NCO group in the hardener to cure.

Lets say our clearcoat has 5 of these OH groups (example only)

Hardener: The hardener has NCO groups. This again is measured on the solid content of the hardener or Isocyanate for the techies.

Lets say our Hardener has 5 of these NCO groups too (again this is an example in simple terms)

We need them 5 groups in the Clear to match the 5 groups in the hardener so we can get a correct cure. If you was to use a hardener that only has 3 groups instead of 5 then those other 2 groups that can't match anything they will kind of just sit around doing nothing (that you know as staying soft) There are other aspects that can cause softness or trouble curing such as the chemicals used in the clear or hardener. It can also work the opposite if you have too many NCO groups and not enough OH in the clear.

There may be some of you wondering how a certain clearcoat ending in 0200 which is classed as a HS be used with the MS hardener. It is kind of cheating in my eyes, many companies will do this but i wont say which ones. ( If you take a look at who uses the same hardener for their MS clears and HS this is a clue) how they do this is they use a clear or resin that has very low OH groups but high solids resins so it requires less hardener. you could get away with this for HS clears but definitely not UHS clears as you do need a lot higher isocyanate content for them than you would MS or HS. Also, you may find that a lot of these clears are not fully VOC 420 g/l compliant and are probably "Anti-Scratch".

So my last look at hardeners is how it can sometimes work out quite well for you when you use a different hardener. This could be that the OH and NCO groups are within a close proximity and the solvents that are used are giving good results. This is on some occasions and you can never be sure that you are getting the correct cure. So the best thing to do is try and stay clear even if it does work!

I hope this brings a bit more understanding of the backbone of these chemicals you are working with. After all, there is a lot of R&D and chemistry that goes into making a clearcoat for someone to just bang another hardener in!

There are a few other things that can cause issues but this is just one thing i come across, feel free to message me if you have any other questions.

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Dani Mattock
Dani Mattock
Jun 12, 2021

So dose the flash time of the hardener depend on the full hard cure of the paint so if it flashes to fast will it fully cure


Sep 12, 2020

I've done this myself when I needed a fast hardener, added some upol 20:31 to my roberlo 610, it was a very cold morning (around 5C in my booth) but all I had was the KX45 but really needed the KX46, it flashed off so quick it didn't have time to flow out. Maybe I just invented super fast clear, I might call it hyperclear! 🤣

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