So you're probably reading this because you've either not used UHS clearcoats yet and want to know what they're all about or, you've tried UHS clears and cant get on with them. Whichever one it is or if you're just looking for some general information hopefully you may take away some little nuggets of information from this post!
What does UHS mean?
UHS means Ultra High Solids. This means that the solids content is pretty much maxed out before the paint or coating becomes un usable (not sprayable).
What classifies as a UHS?
Unfortunately this is my biggest pet hate in the industry because there is not an official classification between MS, HS and UHS. Therefore some manufacturers take advantage of this marketing method and choose to call their clearcoat a MS, HS or UHS what ever they decide to market it at.
This issue is actually a big issue and I have personally tested some clearcoats that are selling as a HS but really match the solids content of an MS. Unfortunately the average painter will not pick up on this.
My general theory of solids classification is,
MS = up to 45% solids
HS = up to 55% solids or 420g/L VOC
UHS = 56% and above
Again this is my theory and not an official classification however i believe this to be a good reference when looking at the solids.
VOC and solids content are similar but not the same in terms of referencing the solid content. Keep in mind the VOC of some solvents can be as low as 600g/l meaning 1litre of solvent only weighs 600grams this means that the VOC content is 600 grams per litre before we have even developed a clearcoat yet so low VOC doesn't always actually mean higher solids and better quality product. This just means the paint manufacturer has carefully developed a product using key low VOC or low density solvents. I can go deeper into this however were not learning how to develop paints here!
Key characteristics of a UHS
Dry Film thickness
We've touched on solid content, this means that when the product is sprayed out it leaves a solid behind (solids cant evaporate). The solid left behind is the cured finish. As the solids are higher with UHS they need to be sprayed different. lets take a look at what dry film thickness is.
The final dry film thickness of a clearcoat is usually around 50 microns (that of a human hair) not a lot right?
The method of applying the clear to get that thickness changes. See below
MS clearcoat at 2 full coats for example would probably be slightly lower than 50 micron hence why you probably don't get excited by the depth of gloss it gives.
HS clears usually need 2 full coats or in some cases a grip and rip as they call it or (3/4 coat and 1 full) this would give a 50 upward micron layer (a deeper gloss level)
UHS is where things switch up. In some cases, 1 full coat could give the same film thickness as what 2 or even 3 coats of an MS would be! and this is where some painters can run into issues because they over apply the UHS which in turn, causes defects later on.
Viscosity
This plays a massive part of what finish you get!!
A MS clearcoat usually is very thin or (low viscosity) this makes it sprayable all year round however, UHS because the solids are so high, is affected by temperature ALOT.
When a UHS is cold, say close to 0 which the UK isn't far from half the time it can be very thick (high viscosity or more viscous) this means that you just wont be able to spray properly and that if you manage to leather it on you'll probably get runs when it gets warm.
This means you need to make sure that the clearcoat is at its optimum spraying viscosity. This can be done by making sure the clear is warm enough to reduce the thickness or by thinning to the sprayable viscosity time.
My recommendation is firstly warm the clearcoat up to room temperature (22 degrees) and no more! in some cases, warming the clear up too much will cause a layer of condensation on the panel when sprayed and cause the clear to drop back etc.
Condensation is created when warm air collides with cold surfaces - warm clearcoat on a colder panel will be the same method. Always make sure the clearcoat is close to your panel temperature to prevent this.
When you have warmed your clearcoat up, a quick viscosity test will enable you to see what you're working with. Then from here use thinner to tweak to the recommended time. Please note! using a good quality thinner will not affect the performance of the clearcoat!
OK so now we have looked at application and the actual mixing of the clear, what else can we do to make sure you get fool proof top quality finishes?
Spray guns
Selecting the correct spray gun when using UHS clearcoats is more critical than ever and this is due to UHS clear technology being so different to say an MS.
What gun do we look for when applying a UHS?
It needs very good atomisation - usually an RP, HTE or similar type where the overspray is more "foggy". This type of gun breaks down the clear better and lays the clear down flatter.
It needs to not overly apply the clear but at the same time not have a too small setup that the clear doesn't come out fast enough - If its this one it may be the viscosity we spoken about but you may want to switch between 1.2 and 1.3 setups.
Transfer efficiency - The spray gun must be able to adequately transfer more product onto the panel than in the air otherwise you are wasting product.
Check out the manufacturer recommendations for the product you are using for a good starting point.
Finally, if you haven't used UHS before please be willing to have a play with it and experiment with what suits you best. I can guarantee once you find the process that works for you, you will absolutely love it!
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