TGIF: Drips, runs and errors Tuesday, November 28, 2017

Runs and drips make your finish look bad.

Avoidance:

Lighting.  

If you are working on a horizontal surface, use a back light at a low angle.   For vertical surfaces back light and inspect at a low incident angle.  Both will help you detect runs and heavy spots (and missed spots).  It also helps to leave backs off casegoods until the insides are finished for better lighting. http://thepatriotwoodworker.com/topic/21059-tgif-finish-then-assemble-or-assemble-then-finish/

Application technique

If you are brushing or wiping, start before the edge and go off the opposite edge.   Come back and go off the edge your miss.  If you are using a slower-drying finish such as oil-based varnish, follow the application with "tipping off."   To do this, wipe the brush with a rag and while holding the brush nearly vertical, make long strokes along the grain.   This can fill dry spots and cut heavy spots.

Most varnishes are pretty viscous right out of the can (probably to make them comply with VOC).   It generally helps to thin them at least a bit (10%) with mineral spirits.

If you are spraying, start the spray off the edge and continue off the opposite edge.  Keep the gun/aerosol can moving.  Work on overlapping each pattern about half. 

There is a device called a "wet mil gauge" that helps you measure the wet film thickness to see it is within the specs for the product you are applying.  Once you get a feel for it, you can probably judge the right thickness.

I think the two biggest mistakes people make when applying varnish is (1) putting it on too thick and (2) not sanding between coats.  Too thick causes runs and brush marks.  Sanding out removes problems such as dust marks and uneven and thick spots.

Correction

In either case, if you catch the drip/run while it's still fairly wet, you can cut back the thick spot with a brush.   If it's started to cure/dry leave it so as to not make it worse.

Repair

If you don't catch it in time, you will have to wait until the area is fully cured.   This can take days for thick layers of oil-based varnish.  Then cut back the high spots with a scraper or sandpaper.   You will then need to apply another coat of the same finish.

Edited November 28, 20178 yr by kmealy