View Full Version : urethane "wave"


05-15-2011, 06:17 PM
I thought I'd start a new post just to address this issue. In my first post (no answers) I mentioned a car my painter just sprayed, a 70 Jag XKE. The body work was flawless. He applied 3 coats of black base, then 2 or 3 clear using matrix brand products. We color sanded using 1200 and then re-shot just clear. It looked really really good after re-shooting, but the painter wanted to put one more coat on and bam, that is when the urethane "wave" showed up. The color sanding, buffing and polishing made the paint shine and depth look awesome but the wave is still there, mainly in the passenger side - side panels.
I found some old posts back in 2006 that touched on this issue. Any updated info from you pros ?

Phil V
05-16-2011, 09:58 AM
Urethane wave is a pain in the butt and no quick fixes. It has to be hand blocked/wet sanded with a flat sander longer than the normal rubber hand block etc. I hand made my sanding block specifically for urethane wave out of piece of slat from a wooden blind and a piece of 3/4" x 1" pine. The piece of wooden blind slat is around 3/16" thick and around 10" long, the 3/4" x 1" pine handle around 8" long and screwed as well as water proof glued to the slat which made the sander rigid to specifically make sure that when sanding it doesn't just follow the contour of high and low spots (trims the high spots only). Along with that sander I use 1000 grit wet sanding paper. It works great for urethane wave.

I have seen a couple causes of urethane wave. Obviously regardless of other associated other causes the main cause is just too much paint - too fast. But an associated cause I noticed was if the ambient air temperature is cool or the paint is cool or the car is cool it definately adds to a urethane wave problem where in the hot summer time that problem wouldn't happen. I've suggested several times here on this site that urethane clear and single stage urethane are very much affected by cool temperatures, it causes the clear to run MUCH easier if the air, the paint itself or the car is cool or cold. As the urethane gets cooler it seems to thicken up in the paint gun or as its being sprayed through the air and also if its being sprayed on to a cool/cold car. cool urethane paint coming out of the gun does not atomize nearly was well as warm urethane so it doesn't go on the car and flow out like it would if the everything was warm.

06-16-2011, 06:59 AM
I am only a back yard painter, but have done some outstading, and FLAT, jobs on my resto's.
As mentioned, I and several other Pro body guys I talk with about urethane wave all agree one one thing. The initial sanding must be done with something hard. I have made my sanding blocks and sticks from maple, cherry, big Home Depot paint stick's and plexi glass strips work great too. Basically you are machining the surface flat, after the first sanding, then you can move on to hard rubber blocks. All my not agree, but we often start with 800 paper too.
The problem I see when starting with even a hard rubber block is the block gives some under pressure and just rides over the wave or puddle, it will come out shiny, and look like cottage cheese, you see it all the time at shows.


06-16-2011, 08:11 AM
Cutting modern paints requires sharp sandpaper and hard long blocks if you're going for a really straight final outcome. Fortunately, it sounds like you've got plenty of paint. You could even start with 800 and a paint stick and treat it like you're blocking primer, just don't go all the way to complete loss of shine, stop where the paint just starts to dull in the low spots, then go to 1000 and go a pass or two past where the paint is dull. Stick with hard blocks up to 1500 then refine your scratch all the way up to 2500 or even 3000. Buffing at that point will be easy and fast and you won't have any shined scratches.


06-16-2011, 11:40 AM
I have done a lot of testing to see what gets rid of wave completely,
I don't get rid of it unless I sand first with at least 600 grit.
400 does an even better job, even with a soft block, 400 will
get you that real flat finish.
Above 600 I can usually still see a little.
My buddy that does award winning show car finishes starts with 320.
And it's not just the grit but how you sand, long strokes from panel
edge to panel edge. It all makes a difference.
Try it and you'll see.