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-   -   Slightly rough after spray painting (http://www.diychatroom.com/f4/slightly-rough-after-spray-painting-72956/)

colinmc8 06-06-2010 09:52 AM

Slightly rough after spray painting
 
Hi,
Firstly, Ill just say Im a Carpenter/Joiner and mostly make custom made furniture.
Ive been meaning to join a forum for a while now for advice on spray painting.
I have been using and self teaching myself with a compressor and HVLP spray gun. It took a while to figure out how to get good results. Im pretty happy now with my results, however I cant seem to get it perfect.
What I usually do is after priming give a light sanding between coats. After a few coats and after one of the light sandings the surface feels perfect, although you can see its been sanded, so it needs one more coat. But then after the last coat it feels slightly rough again! but looks fine. Do I need to give a very light sanding after that last coat? (is it normal practice?) or would you say its something to do with dust in the air?
I appreciate any advice.
Colin.

Matthewt1970 06-06-2010 12:14 PM

I am not a furniture finish expert, but I think your final sanding needs to be with a 600 grit or buffing pad.

colinmc8 06-06-2010 01:23 PM

Thanks Matthew, basically the question is, is it normal practice to sand after the final coat is applied or is it normally left as is? Oh and I'm currently doing a painted gloss finish, solvent based.

Rcon 06-07-2010 03:09 AM

Quote:

Originally Posted by colinmc8 (Post 452071)
Thanks Matthew, basically the question is, is it normal practice to sand after the final coat is applied or is it normally left as is? Oh and I'm currently doing a painted gloss finish, solvent based.

Using a compressed air sprayer? I'd guess that your product is drying in the air before it hits the surface - you need a retarder. Can't suggest one though as I don't know what product you're using.

colinmc8 06-07-2010 04:04 AM

Thanks for your reply, I have never heard of retarder before, just done some reading in the subject. I do need to thin the paint quite alot for it to come out the gun smoothly. Does thinning the paint cause it to dry faster in the air? or is there any other way to prevent this, I would prefer not to have to buy more products.

mark942 06-07-2010 06:20 AM

Over thinning paint will cause you problems. Do you have the right needle for your particular application? When you say solvent based material, do you mean Naptha, Lacquer, MEK, etc? What ratio mix are you using? What grit paper are you using between coats? Do you size up grit between coats?


Yes, sanding between coats is necessary. :thumbsup:

colinmc8 06-07-2010 06:38 AM

Hi mark,

All of the questions you just asked are the reason I joined a forum lol I don't really know is the quick answer!
Lets see I'm using gloss paint and thinning it down with mineral spirits, I don't do exact measurments for the mix...maybe I should? probably about 1 part mineral spirit to 2 part paint.
The needle is 1.4 (Thats stamped on it. I don't know if that helps)
Between coats I use a sanding sponge I guess its around 240+ grit.
Just one quick question, after the last coat, do you normally just leave it or give it a very light sanding or something else?
Thanks.

mark942 06-07-2010 08:46 AM

http://www.advmachinery.com/c-40-750...les-chart.aspx


I copied this for you to look at. Reinforces the reducer comment.
Is thinning necessary or important with HVLP? Viscosity of coatings is important. Although we supply a guide, there is often some trial and error involved in arriving at the best viscosity. If a product is thinned too much, there are runs. Too thick and 'orange peel' or rough finish is the result. When thinning, it is essential to use a reducer that is compatible with the product you are using. To be sure, buy a thinner/solvent made by the same coatings company - always verify that it is the right product. It is wise to experiment on a practice piece to ensure that the finish is perfect. You may also request information from the coatings manufacturer - don't forget to mention you are spraying with HVLP equipment.



Try starting with your 240 then move up to 1000, in some cases I have used 3200, the finer the grit the smaller the scratches it will leave. If it is a very very fine finish your looking to get, after sanding try using wax after all spraying is done. Then hand rub it to a glass finish.

colinmc8 06-07-2010 11:33 AM

Thanks alot, your help and time is much appreciated. In the end I just gave a light sanding after the last coat and it is fine. I will remember your advice for future projects.


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