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  #1  
Old Mon 31 August 2009, 04:22
Alan_c
Just call me: Alan (#11)
 
Grabouw (Western Cape)
South Africa
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Seeing as though you do so much MDF work, any tips on sealing the cut edges so that it takes stain evenly and does not keep sucking up all the lacquer applied????
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  #2  
Old Mon 31 August 2009, 05:02
hennie
Just call me: Hennie #23
 
Roodepoort JHB
South Africa
A moerse lot sanding sealer with not a lot of thinning with thinners.1 st coat must be thick
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  #3  
Old Mon 31 August 2009, 13:16
Jan de Ruyter
Just call me: Jan
 
Pretoria
South Africa
MDF is THIRSTY!

The following is not an advertisement for any product but the result of many years of searching for the right (read: easy!) solution.

I restored old pianos for many years and the product that worked best is a twin pack catalyst lacquer. The catalyst cures the lacquer so when a second coat is applied, it does not create that sinking feeling. After using good old Brummer stopping to fill the big holes, small blemishes can be filled using a small drop of lacquer. The product I use is Turbobright. It comes in gloss, satin and matt and can be tinted to any colour. The satin gives the best finish as a blemish is not that visible when using gloss while it is not as dead as the matt finish. It is heat, UV and water resistant although you cannot throw it down a mine shaft without expecting any damage.

Any successful spray job starts with preparation. If this is not done properly, no amount of spray will fix it. Start sanding with 80 grit paper. For big surfaces, I use a belt sander. When sanding veneered board, I only use hand as a belt sander is far too quick and will leave you with a nicely sanded substrate. Small areas are done by hand.

After 80 grit, I sand with 220 grit free cut paper. Pre-sanding the MDF edges with 220 free-cut paper solves this problem to some extend, saving you litres of lacquer.

In addition, there is a filler you can apply to the edges of MDF with a brush. Sand it down before applying the lacquer. Available from Technipaint.

An orbital sander tends to clog up, causing little worms to appear. This is caused by residual resin in the wood heating up which then sticks to the sand paper.

I spray using a Devilbiss high pressure gravity feed gun, thinning 2/3 lacquer with 1/3 thinner. The first coat is a liberal coat, using 50% overspray to ensure full coverage. Use the lowest possible acceptable pressure (30psi, sorry, old compressor) to avoid spraying your expensive lacquer into the air.

First coat is sanded with 220 free cut paper. Normally I use an orbital sander for this as you must try to only remove the humps and bumps and leave the surface as flat and smooth as possible.

The second coat is applied thinner and sanded lightly.

You can use as many coats as it takes to fill the grain. Oak, kiaat and mahogany is particularly open grained and may require a few coats.

The last coat can be denibbed using 1000 grit water paper and H2O. On wood, I apply a coat of Cobra furniture polish to give it a nice, smooth, baby bum finish.

The problem with using sanding sealer is that it remains soluble with thinners. As soon as you spray the second coat, the first coat (even if dry) becomes a liquid again, sinking into the MDF even further, making it an endless, thankless job.

Shellac is not water proof and you have to use methanol (wood alcohol), so you might get high on the stuff. It also requires a lot of elbow grease (available at your local hardware store). It also tends to clog the sandpaper very quickly and is not water resistant.

Treating any wood with oil is not recommended. Oil does not provide adequate protection against moisture as it leaches out of the wood over time. Once treated with oil, there is little you can do to salvage it again except to apply MORE oil.

Some people use a silicone spray on wood surfaces. This make restoration nearly impossible as the silicone reacts adversely with any lacquer, causing fish eyes to appear.

For those living in sunny SA, contact Newchem Industries , they manufacture Elvolac.(used to be Dulux Industrial Paints) on 031-465 0365, they will sort you out in no time.

Technipaint is the distributor of Elvolac: http://www.technipaintdbn.co.za/indu..._coatings.html.

For those not in sunny SA, try an industrial paint supplier, as DIY products are normally insufficient and the salesperson wants to sell you as many tins as possible without providing any technical knowledge.
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  #4  
Old Mon 31 August 2009, 23:00
Doug_Ford
Just call me: Doug #3
 
Conway (Arkansas)
United States of America
Wow!! You guys are certified experts. Thanks for the info.
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  #5  
Old Mon 31 August 2009, 23:56
hennie
Just call me: Hennie #23
 
Roodepoort JHB
South Africa
I knew someone was quicker to type than me
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