Priming mdf

[bob lawrence]

I need a bigger layout, 6'x 30", now this may not be very big to some but it's big enough for me to have to build it in my shed. The baseboard will be 12 mm MDF, suitably re-enforced with timber.
As it's MDF in a shed it will need to be sealed so I would like some ideas what to use please.
The internet has confused me, which doesn't take much, so I thought I would go to those who know.
Thanks

[ntpntpntp]

In the past I've simply used PVA, diluting it a bit by dipping the brush in water as I go.

[jrb]

If it's going to be in the shed, then using standard MDF at all is far from the best option. It'll be subject to huge temperature and humidity variations.

I'd suggest switching from standard MDF to moisture resistant MDF (MR MDF, identifiable by the fact it's dyed green). It's available from most timber merchants and board suppliers and will cost only slightly more than standard MDF (last time I bought some was in January, and I paid £14.50+vat for an 8'x4' sheet).

Alternatively, a good quality 12mm WBP plywood. WBP stands for "water and boil proof" and refers to the type of glue used to bond the layers together. Expect to pay about £20+vat for that.

Either of those options will be far better than standard MDF, but you (probably) won't find either at B&Q. Go to a local timber yard instead.

[daveg]

Strongly endorse jrb's recommendations.

Please don't use 'standard' MDF as heartache will surely follow!

I'd varnish every face of all materials used to help prevent damp getting through.

Hope to see what you get to build in your shed.

Dave G

[NeMo]

If it's going to be in the shed, then using standard MDF at all is far from the best option. It'll be subject to huge temperature and humidity variations.

I totally agree with this 100%. MDF is widely used indoors without varnishing. All sorts of flatpack furniture for a start, as well as off-the-shelf layout kits. My wife primes with PVA when doing her MDF craft kits and is going to decoupage them.

If you need to varnish MDF, then you shouldn't be using MDF. As stated by @jrb above, something like marine plywood makes infinitely more sense, and even then, you'd want to brace it thoroughly to prevent warping.

Cheers, NeMo

[Steve.T]

My personal preference is also good quality 6mm plywood, with of course all the necessary bracing that goes with it.
If the layout was not portable I probably would up it to 9mm.
And certainly for a layout in a shed (even a well ventilated, insulated and heated shed) I would go for good quality marine ply.

However I know there will be as many replies saying they have used MDF for years and had no issues and like everything else a lot is up to personal preference,any maybe well braced MDF that is well sealed may be perfectly satisfactory.
But, if you think marine ply would be better and are going for MDF to save money I would definitely go for the ply option, save all the hassle of two or three coats of yacht varnish and ruined brushes and save the money on something else. After all if the baseboard gives you grief later on it will be a nightmare.

Again my personal preference.

Steve

[ntpntpntp]

Don't think I'd go for MDF for main baseboards, certainly not in such an environment. 

I've always used 9mm ply for baseboard construction, at least as the outside "box" for an open-frame baseboard.  The layout lives in a garage in between exhibitions so has to put up with temperature variations, though not to the degree it might in a shed.

I've used MDF for two small fiddleyard "boxes", as an experiment rather than any definite desire to use this material. The MDF was sealed with PVA and painted. They seems to have lasted ok so far (coming up to 10 years old).

[bob lawrence]

Thanks for all of the replies, unfortunately I already had the sheet of mdf, cut to size so needed to use it. I chose to go with the Wilco quick drying matt varnish, giving the board two coats, both sides, especially the edges. I probably would have gone for ply, had I posted on the site first, but it's done now so fingers crossed it will be ok also with it being a bit more substantial at 12 mm. Have just been used to using MDF so just went with that, it could quite easily have been thinner i.e. 9 mm. The smaller layout this replaces is in the loft and will be dismantled shortly, there is also a fairly large oo gauge layout up there so I will be able to get around easier.

[njee20]

I'd suck it up and buy decent ply now. It'll be a far more expensive problem to resolve when the whole layout warps. Sure the MDF will be useful for something.

[NeMo]

Thanks for all of the replies, unfortunately I already had the sheet of mdf, cut to size so needed to use it...

Well, good luck! This is one of those times when heavy duty bracing with lots of screws is probably worthwhile (I do think we tend to overdo the woodworking when working indoors a lot of the time). If it's properly secured with some good quality lengths of wood so the MDF can't move about too much, you should be fine.

Just a thought, but with summer not too far away with its potentially extreme temperatures, it might be worth laying the track but holding back on any expensive scenics for a while. See how the track holds out for the next few months, and if everything works the way it should, then that's a positive sign!

Cheers, NeMo

[bob lawrence]

Thanks for some encouragement to continue. I appreciate all concerns and will certainly 'overbrace'. I am still in the planning stage, using the back of a suitably sized piece of wallpaper and have already needed to relocate a turnout where the edge bracing will go. Once I am happy with the design and have located all turnouts, considering point motors, I will add as much bracing as I can, drilling through for bus main etc.
Thanks to all for your input.

[DELETED]

Well just to throw a little into the balance.  I finally got started on a proper layout and used batting from BnQ (for every 1 I found I probably rejected 10).  I have also used relatively thin MDF rrom BnQ.   15 months later no change or warping and I did not seal the underside (which I would not recooment by dilluted PVA as backed up by previous posts on here) -quote surprised as it is my "drying room" so te wet clothes go in there and I run the dehumidifier often.

I think the books describe the principles of baseboards best -any deviation is pretty much down to the person.

Rich

[Ian Bowden]

I have been following Everard junction on YouTube for some time although it is OO (sorry for the swear word) there are plenty of modelling techniques on show. He has just ripped up 7 years work because of warping on the mdf sections of his baseboard causing derailments all the time

[longbow]

The cause of the warping is more likely the framing than the baseboard, especially if it's a timber frame. An MDF baseboard with a warping tendency is not going to pull a solid frame out of true.

The main reason for MDF warping appears to be temperature or moisture differentials between the faces which would suggest that sealing the board is a good idea.   

[njee20]

Well just to throw a little into the balance.  I finally got started on a proper layout and used batting from BnQ (for every 1 I found I probably rejected 10).  I have also used relatively thin MDF rrom BnQ.   

Was that not enough of a clue?! Never get your wood from a DIY shed, go to a timber merchant. It'll be a similar price, but vastly better quality.

The cause of the warping is more likely the framing than the baseboard, especially if it's a timber frame. An MDF baseboard with a warping tendency is not going to pull a solid frame out of true.

Except if the MDF starts to split, it'll just pull itself away from the frame.