Spraying Polyurethane

[cosmoad]

Can anyone advise what is the best thinner to use when spraying Polyurethane so that I can eliminate or minimise runs and sags. I read somewhere that by using a thinner that evaporates quicker will help. ie Acetone

[echnidna]

Read the can.
If you mix incompatible thinners it wont work and might even clog up the gun.

[durwood]

Hi Cosmoad,

Echnidna is correct you must use the correct thinner, the faster the thinner dries the less gloss you will get. The wrong thinner will not mix properly with the paint and you will get other problems such as non or unever drying, soft finish even lumps in the paint, if you get it really wrong you may get the paint to turn into a thick lump instantly.

Depending on the type of poly you may be able to get fast or slow hardener and thinner for it. You can use fast in cold weather to help drying time or you can use it if the area is small. On large areas or in hot temperatures you may need to slow down the drying so you don't get dry areas from the paint drying before you get to spray the next pass onto them,

Don't want to get runs??

1) follow mixing instructions from manufacturer.
2) use a gun made to spray the poly. ( for enamel spraying)
3) check the viscosity ( how runny the paint is ) a viscosity cup is used to do this, you can buy cheap plastic ones. They are filled up with the mixture and then the time is measured for the cup to empty through the hole in the bottom.
4) adjust the gun correctly. Open the fan to wide, that means turn the control that lets air out of the holes in the horns on the air cap of the gun to maximum. turn the knob behind the needle in till the paint won't come out and then slowly open it until it just lets out enough paint to get a wet continuous coat at the distance and speed you spray at.

Practice on something before you attack something that matters, and make sure you WEAR A MASK and are in a well ventilated area.

Don't spray when the temperature is below 15 degrees C. if its too cold the poly will not dry. If you want to spray a second coat leave it for about 10 minutes, if you want to do more coats or you have mixed up too much paint and want to use it after rubbing it back. Tip it into a tin and then a plastic zip seal bag and put it in the fridge till the next day. Most two pack mixes will stay liquid for 24 hours, Wash the gun out before 6 hours whatever you decide or you may not ever get the poly out of it.

Hope that help.