Seal MDF Spoilboard

[bakerkr]

I read an article that talked about mixing 1 cup glue with 10 cups of water to make a sealing agent for MDF. I just glued a new spoiboard to my bot and wondered if AFTER I surface it to put this mixture on to add more stiffness. I don't use a vac.

Kevin

[ed_lang]

I would not want to add any water to my MDF spoilboard and cause it to swell.

[davidp]

Kevin

PVA glues are water based and as such it is perfectly reasonable to mix it with water and use it the way you suggest.

It will effectively seal mdf after it has been machined. You will need to sand it after it has dried as it will raise the grain slightly but that will vary with the glue formulation used in the MDF production.

MDF is reasonably resistant to water and will swell a lot less than chip board and even some species of solid timbers. The amount of water you are talking about will have no effect other than the grain raising on the surface.

If you do not use vacuum, you will find that sealing your MDF will keep it more stable for longer.

By the way the Health problem with MDF is the particle size of the dust and not necessarily the Formaldhyde. You will ingest more formic acid from a can of diet coke than you will from surfacing a sheet of MDF, so it is essential to use a good dust extraction system and to wear a dust mask when cutting, sanding or handling any MDF or indeed any timber.

Happy botting

[ed_lang]

David,
I had no idea that MDF would accept water like Kevin spoke of and not swell up. MDF seems to swell up from just having high humidity around it.

[davidp]

Ed,

I agree that MDF will expand and contract with humidity but then so will all timber, and it WILL swell if left in contact with water for long periods.

I know the formulations for the glues in MDF are different in each country and you use brand names that are unfamiliar to me so I am unsure if we are talking about the same MDF. We are able to buy a moisture resistant MDF (MR MDF) that is great for use in kitchens and particuarly bathrooms. It will take enormous amounts of moisture over prolonged periods before showing any signs of swelling.

Here is a little experiment to try.

Cut a piece of 18mm(3/4) MDF (Standard MDF not the MR MDF) and a piece of 18MM Chipboard each about 450mm (18") long and 90 mm wide then get a few pieces of solid timber each 18mm thick and about 450mm long.

Place the end of each test piece in its own container and then fill the container with water. Leave the pieces in the water over night then perform two measurements, 1st the amount of water sucked up by each test piece and then the amount of swelling of each piece.

You will be surprised by the results most people are.


Regards,

[dray]

I was gonna chime in on this last night. David is 100% correct. Alot of people are afraid of MDF as I was about 10 years ago. I thought I was being a purist using only shop birch and poplar etc for paint grade, calling MDF users out for using sub standard materials.

But boy was I wrong. For some reason people associate MDF with an undersink piece of swollen and rotten particle board they tore out with their dad in the 70's.

Mdf to me is the best stuff to work with for my cabinetry. It takes paint incredibly well. It doesnt swell anymore than a piece of shop birch does if left in water. Its not even an issue to me anyways.

It just has some basic differences and people have used ply sheet goods for a hundred years and are afraid of change.

For milling and stability there is only MDF in my book.

Oh yeah it doesnt have anymore formaldahyde than your favorite ply goods either. But milling releases smal wood particles around 30 microns so a good vacuum setup is a must.

[fleinbach]

I started using MDF around the late 90's and grew very found of it quite quickly. I too noticed that it handled moisture very well when a scrap piece setting outside the shop exposed to several rains showed very little swelling. So I started using it on the exterior of my additions. Here is a link to a porch made mostly of MDF. http://homeimages.net/images/Pannella/Pannella%20Finished_0638.JPG. This was not the water resistant type but standard off the shelf stock from Home Depot.

The picture was taken a few months after it's completion and the date stamp shows it has been up for 6 years. I went past to see how it held up last year and it still looked in perfect shape after 5 years .

The theater rooms I build are almost entirely made using MDF. The simplest ones require about 8 to 10 sheets and the most complex one contains over 36 sheets.

All this MDF requires painting and I've tried many methods to seal the edges and surfaces of 3D pieces. I also have used water thined glue which works well to seal but wasn't the easiest to sand. The best thing I have used is called Ready Patch. It looks like and has the consistancy of standard drywall compound but dosen't shrink as much. I fill all nail and screw holes with it. For edges I thin it with water and paint it on.

[john_l]

Frank, are the fascias in that photo mdf also? That's just amazing to me. I had replaced some carved signs made by another sign shop years ago that were made from, what looked like to me, mdf. These were replaced because of excessive swelling near the edges where the paint had let go and the water could get absorbed. Anyway, it scared me so that I've never used mdf outdoors, even under a canopy. Maybe the mdf is made differently today than was done in yesteryears? Thanks for posting!

[myxpykalix]

I have no experience with mdf outside but this is a tip Ed L gave me about sealing the edges of mdf to keep vacumn in. If you take clear silicone caulk and apply a thin layer to the edges it dries to a rubbery seal and doesn't let air out which means it may not let water in. Just a thought.

[fleinbach]

John,


No, the facia is not MDF but the front curved piece is, and even though it is under the roof it still gets wet lots of the time from wind blown rain.