[rwskinner]

I plan to use two pieces of 3/4 mdf for my router table, one of which will be my spoilboard and the other as my base with tnuts. Since I'm in a high humidity area would it be wise to seal these to resist swelling?

[wizard]

Yes!

Unfortunately I don't really like MDF for this very reason. It isn't just humidity changes that are a problem, spill the wrong fluids on the table and you will get swelling and distortion. A lot of effort goes into building these machines thus you really need to protect that investment in time and money. the little bit of extra effort required to seal or otherwise protect the MDF structure will pay off big time if unforeseen crap happens.

[rwskinner]

I was kind of thinking that way as well. All my searches pretty much brought me to folks using MDF for the vacuum table so they sealed it.
Is Thompson Water Seal okay for MDF?

[ger21]

My opinion is that you don't really need to seal it. Unless you actually get water on it, any swelling will be uniform, and fairly minimal (a few thousandths).
Since you'll be zeroing the tool each time you use it, variations in spoilboard thickness should have no effect on your cutting.
Also consider that you'll periodically want to resurface the spoilboard, which will result in having to re-seal it.

[rwskinner]

I was mainly thinking about sealing the more permanent base board than the spoil board on top. Honestly, Most of my projects will be fairly small at first so I'll most likely clamp down a smaller spoil board on top of the two 3/4" boards to preserve it as long as possible. Later as I become braver I'm sure I will start using much more area to cut on.

I do have a shipping table from graingers which has a nice heavy stand and 1" MDF or Particle board top. I guess MDF and particle board now days is the same stuff?? The top hasn't ever been wet to my knowledge, at least not from sitting stuff on it or spills, and it has bubbles all over it. My shop is a 30x50 metal building and it's not insulated so I have to keep all my machines very well lubricated or they will rust over night. It's mainly this time of year in South Texas were the temperature swings are radical. Sometimes it's so bad everything inside the shop is dripping wet, but that only happens a couple times per year.

I do have a 25x30 insulated and climate controlled shop but it's full (game room and electronics) and I'm thinking of building another one for my cnc mills and lathes, and now, a cnc router. The equipment would survive better and during the summer I would survive better as well! Tax man will love me as well

Thanks for all the input.
Richard

[wizard]

I was kind of thinking that way as well. All my searches pretty much brought me to folks using MDF for the vacuum table so they sealed it.
Is Thompson Water Seal okay for MDF?

There is an incredible number of threads about MDF and sealing it in the various wood working forums. I'd suggest looking on the net to get a broad idea of the various methods people use to seal up MDF. As for Thompsons Water Seal, I have no idea what that is, however if it is water based I'd say stay away from it.

Some methods I've seen described:

1. Boiled Linseed Oil. {make sure you understand the risks}
2. Epoxy thinned with Alcohol
3. Thinner Epoxy formulations.
4. PVA glue thinned with water.
5. Danish oil

There are a bunch of others! How important this sealing is probably varies with individual circumstances but for most usage I see it as vital.

This material soaks up fluids very fast, sealing effectively prevents or at least slows that process down. In fact in almost every case the applicator needs to apply several coats in quick succession to make sure the "sealant" soaks in completely. By the way, ideally you would seal the parts of your CNC completely, inside and out. A good sealing technique will actually hardened the material a bit.

As mentioned before I hardly use the stuff as my experiences have not been good at all with MDF. It might suffice for indoor well protected projects but in my estimation it isn't worth the grief. Yes I know it has some characteristics that make it ideal for building CNC machines considering its cost. I've just seen the material degrade many times well before the expected lifetime.

Now admittedly this can be partly blamed on quality control, some brands are better than others. So if you go this route at the very least by a well respected brand and not some no name import. Second consider the so called moisture resistant products that are now on the market. Third make sure you buy from a reputable dealer.

The last time I used anything similar personally was a project using particle board a close cousin of MDF. In that case I covered the surfaces with Formica and the edges with solid wood. This is an alternative but probably not practical on the structure of a CNC machine. In this case I had no choice but to seal because the particle board would effectively soak up much of the glue.

[rwskinner]

Thanks Wizard. I may have communicated incorrectly. My router terminology isn't very good at this point. The machine itself is built from aluminum extrusion and steel. Just the cutting board/spoil board and what it mounts to would be MDF.

Thanks again for the insight.
Richard

[ger21]

I guess MDF and particle board now days is the same stuff??

No, they are two very different products. It sounds like your table is actually particle board.


Now you're talking about condensation, which is a lot different than just humidity changes. Condensation doesn't normally occur on wood, but if there's any chance of water dripping on to the table, than yes, I'd seal it. I'd just get a quart or gallon of polyurethane and brush a heavy coat on. Most of it will soak into the MDF, but it should be sufficiently sealed.

[wizard]

Thanks Wizard. I may have communicated incorrectly. My router terminology isn't very good at this point. The machine itself is built from aluminum extrusion and steel. Just the cutting board/spoil board and what it mounts to would be MDF.

Thanks again for the insight.
Richard

Oh. That is a huge difference. Depending upon your processes and mix of components you could be replacing that spoil board often, there is little reason to seal it in that case. However if you have well defined parts to process along with fixturing to hold parts that spoil board might last for a very long time, in this case it might pay to seal it. In general though spoil boards are expendable items that eventually have to be replaced.

The support board underneath is a different story, I'd seal it simply to protect it for as long as possible.

Another point we have been using the word "seal" here which in my estimation is a surface process. What most people are doing could better be called a filling or infusion process. They are effectively filling the voids in the material by flooding the board with their preferred "sealer" until the board can take no more. The sealant literally soaks in. This can have a very positive impact on the board.

[louieatienza]

QUOTE]The last time I used anything similar personally was a project using particle board a close cousin of MDF. In that case I covered the surfaces with Formica and the edges with solid wood. This is an alternative but probably not practical on the structure of a CNC machine. In this case I had no choice but to seal because the particle board would effectively soak up much of the glue. [/QUOTE]

Normally, when glueing laminate to MDF for particleboard, one coat of rubber cement would be spread down and let dry, then one coat on the laminate and a second on the substrate (especially the edge), to get a good adhesion.

[wizard]

Normally, when glueing laminate to MDF for particleboard, one coat of rubber cement would be spread down and let dry, then one coat on the laminate and a second on the substrate (especially the edge), to get a good adhesion.

Actually I used a contact cement which i guess is rubberized. Even so it was pretty obvious that the porous board soaked up some of the cement.

Interestingly this was for a large router table I built years ago before I even knew about low cost CNC systems. I've been contemplating turning it into a combo router table and CNC router simply because I lack room for too many machines. The table needs a bit of work because it has a slight bit of warping in the table top.

[ssutton]

I use 3/4 MDF unsealed for my sacrificial top. I use vacuum hold down so I cannot seal it or I couldn't draw thru it.
The only sealant that I have applied to it is on the sides. I also live in a high humidity area of southeast TN. All the MDF laying around my shop will warp after a short period of time because of the humidity. However, the router top stays nearly perfectly flat because it is bolted down with these.

Attachment 219074


I countersink 1/2 inch deep pockets for these bolts and I have them about every 8 square inches on the table. As Gerry said, the only variations that I get is just a few thousands of the inch and I touch off at every job so I never really see its effects on my work.

Scott