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WOOD and METAL FINISHING (basics)

 

photo of paint brush

spray paint gun and tank

To achieve a good finish takes time. I usually plan on almost as much time adding a finish as I put into the making of the thing. The following seems daunting but if you want a really good finish that lasts, then these are the steps.

WOOD FINISHES (basics)

Purpose of a finish is to protect, enhance and preserve.

Prior to any finish the piece must be smooth, clean and dust free

(unless of course, you wish a textured surface).

1) SAND until you feel no imperfections. Start with coarse paper and move

up to finer grits. You should end at 220 grit before finishing. Always sand

with the grain. Never use steel wool!

 

2) CLEAN! Blow off with compressed air, use tack rag to pick up small

particles. (tack rag for this purpose is a cotton t-shirt or lint free material

lightly dampened with paint thinner). Make sure your hands are clean when

handling.

 

3) STAIN IF DESIRED. Make sure your stain is compatible with the final

finish you've chosen. Stain 1-3 coats to get your desired color. 3 coats is

usually the darkest the stain will color. Put on with small rag dipped in stain

or a brush. Wipe off excess. Depending on type/vehicle of stain 3-6 hours

between coats.

 

4) FIRST COAT Of Finish. Acts as sealer (in case of transparent finishes),

is primer in case of opaque finishes (paints). Thinned up to 30% for clear.

 

5) SAND LIGHTLY. Sand with 220 or higher grit to take off any raised grain

that the first coat brought up. Do not sand through your first coat.

CLEAN THOROUGHLY AGAIN.

 

6) SECOND COAT. Full strength in case of transparent finishes, little

heavier than first in case of paints.

 

7) REPEAT #5 IF NECESSARY.

 

8) THIRD COAT may be necessary in case of transparent finishes and if you

have paint finish on a complex shape.

 

TYPES OF FINISHES

Polyurethane, oil paints, lacquers, water-based paints, tung oils all

offer choices of sheen: Matt (flat), Satin, Semi-gloss, Gloss.

All have different levels of protection.

 

 

PAINTS.

Oil-based (alkyd) are mineral spirit soluble, water based (acrylic, latex), are

water soluble, lacquer based are lacquer soluble. You will rarely find alcohol

based paints but some primers are. There are also milk-based paints. Each

provides a different; finish or ease of application and durability. Each is cleaned

by the soluble liquid which also acts as the vehicle for the pigment.

 

STAINS.

Are commonly water based or oil based (most common), pigmented and

penetrating. Chose one compatible with your finish coating. The clean-up

instructions on the back of the can will indicate which 'vehicle' (solvent) the stain has. If this is

your final coat it can rub off. Should have a clear coat protective finish. There are also aniline dye stains that are mixed with denatured alcohol but these are more difficult to find and use. They tend to give the most intense color if mixed properly.

 

VARNISH/POLYURETHANE.

Virtually the same thing. Available in water based, brush-on and spray.

Drying times vary. Adds slight yellow color if oil based. (water based does

not). Reasonably oil and alcohol resistant. Brush or spray.

 

 

LACQUER. Quick drying with virtually no color. Will lift (melt) other

finishes (oil and alcohol based). Make sure you use compatible products).

Available in brush and spray. The most water and oil resistant of finishes.

Spray application is easiest.

 

WAX. Once used on something, nothing can go over it and stick. Advantage is that it is

easily repaired and renewed.

 

OIL FINISHES. Many varieties. Most are applied with soft rags.

-Linseed oil. Avoid, as it never fully dries.

-Boiled linseed oil. Soft matt appearance. Multiple coats. Takes a long

time to dry thoroughly after last coat.

-Tung Oil, Danish oils. Similar, although Tung Oil does build up a finish

with multiple coats. Both dry more rapidly than boiled linseed oil.

 

-SHELLAC. Available in clear and amber. Dries very quickly, has a gloss

finish. Is not water and/or alcohol resistant.

 

CLEANING BRUSHES.

 

Buy the best brushes you can for the project. If you take care of them they will last for years. I've been using the same brush to apply polyurethane to refinish the 3-D areas wood tables for the last 12 years (up to 70 tables a year).

Acrylic and Latex Paints.

Best brushed with synthetic bristle brushes. Better brushes have longer bristles and more 'flecking' at the ends (like split hairs). Clean with soap and warm water. If you have dried paint on the bristles towards the heel (nearer the handle) try soaking for a while in warm water and ammonia and then using stiff brush scrub the bristles from heel to end. When done, rinse, use another solution of soapy water to do final clean and then wrap the bristles to maintain shape of the brush bristles as it dries. Store with bristles wrapped.

 

Oil Based, Polyurethane, Stains, and any mineral spirit based finish.

Best applied with natural bristle brush.

I save my old mineral spirits and let the pigments drop out after a few days. This means I don't generate a lot of mineral spirit waste for cleanings.

1) Soak brush in mineral spirits to cover the heel. After 1/2 hour or so scrunch brush down into the mineral spirits to dislodge excess.

2). Second soaking. This time let sit for up to an hour. Repeat the scrunching to get any excess paint/finish out. If the mineral spirits are fairly clear you are done. If not repeat and then shake out.

3)Soak in lacquer thinner 5 minutes of so. Scrunch brush several times. Dispose of lacquer thinner (can't save for later use as pigments don't drop). Shake off excess. Wash brush in soapy water (adds lanolin back into bristles), shake off excess water and then wrap bristles with paper to hold shape as they dry. Store with bristles wrapped.

 

Lacquer and Alcohol based finishes.

Natural bristle brushes best. Cleaned with lacquer thinner and denatured alcohol respectively.

 

METAL FINISHES (basics)

The following refers to painting bare metal. If there is a previous finish it must be in solid, good condition. Unless the surface is 'polished' no sanding is necessary. If it does have a polished feel, 320 wet/dry sandpaper or a Scotch-Brite pad will give the surface some 'tooth'.

 

Clean is the rule again. Wipe down the surfaces with mineral spirits or denatured alcohol before beginning. Make sure there is no wax, oils or silicone on the surface. Even our fingers have acids and oils that can contaminate a finish. A knit cotton rag is best (T-shirt material) as it doesn't shed fibers.

 

-PRIMER.

A primer is necessary. It prepares the surface by etching the metal and will make the top coat stick better. There are 'etching primers' available but they are used mostly for finishes that will be exposed to weather, scuffing and wear, like an automotive finish. Etching primers etch deeper with chemical action and adhere better.

-There are ' high build primers' that are thicker and allow for extra sanding of the primer before the finish coating to get a smooth finish surface where there are minor imperfections on the base metal.

-Any metal that can rust (iron, steel) needs an oil based primer. Galvanized metal requires a specific primer intended for galvanized.

- Aluminum requires scuffing of the surface (Scotch-Brite pad) before cleaning with a solvent to remove surface oxidization.

-If there is rust, remove all of it.... or remove the loose rust and prime with a 'rust encapsulating primer'.

 

BRUSH OR SPRAY.

Because of the flatness of the surface, spraying gives the best results BUT careful use of a very good bristled brush will also result in a fine finish.

You do not need to sand in between coats unless imperfections appear or the finish is gloss in which case a slight sanding or scuffing will give better adhesion for the next coat.

-Wet/Dry sanding. This is done with special sandpaper, dipped in water which keeps the paper from clogging. If you are especially OCD you can go to 600 grit, otherwise 320 is fine enough to maintain a smooth surface.

SPRAY.

Unless the object is large a rattle can will give fine results and is easiest to use and obtain. Read the label to know the drying and reapplying times and stick to it. Make sure you keep a 'wet edge' over spraying i/2 on each pass. Move side to side and top to bottom starting and stopping before and after the edge of the surface. DON'T OVER SPRAY, use thinner coats. Plan on at least 3 (after primer) coats for full coverage

-If you get runs, sand back with 220 or 320 wet/dry paper, then clean and respray.

-Blushing? is overspray that leaves a dull flecked appearance. Means you aren't spraying close enough or allowing a wet edge on the passes.

-Orange peel effect indicates not enough drying time in between or too much sprayed.

-'Fish eye' indicates the surface wasn't clean enough and there is contamination, wax or oil on the metal surface yet. Remove what you've sprayed, and clean thoroughly.

BRUSH.

Make sure the paint is thin enough to 'flow' and not leave brush marks. Thin only with the correct solvent/paint vehicle for your specific paint. Brush in the same direction or drag a wet brush tip across the paint when the surface is painted to eliminate brush marks. Again, don't apply too much on the first coat or you'll get runs. At least 2 coats after the primer to get full color and coverage.

 

AUTOMOTIVE FINISHES.

For an especially durable finish an automotive paint is the way to go. There are acrylic enamels (with hardeners), urethane finishes (with hardeners), and 2 pack finishes. All require special handling and mixing. All require a large capacity compressor, paint booth, professional spray gun (HVLP suggested) and a respirator. The cost is much higher for the paint, thinner and hardener and even purchasing the least amount possible (usually a quart for mixed colors) can run around $100.00.

-Powder Coating. Anything other than very small pieces will require a large oven for setting. Consider using a professional.