MDF - seal before or after assembly

[maxi-model]

After 15 months I've just got started on the first of my Resurgam made SAR kits. This is the first time I have worked with MDF. Question - seal the individual components before assembly ( with either PVA or CA ) or after as complete sub assemblies before priming and painting as usual before final full assembly ? My concern is that the sealer will act as a barrier and my not allow a full bond from any adhesive I may use as a result. Got me a tin of Rustins as advised on here previously, can it be airbrushed ?

Little picture of one major sub assembly. I am amazed at the accuracy of fit of the components and the detail that they contain. Max

 

[Paul M]

If you seal it before assembly, will the glue work?

 

[PhilP]

Assemble first..

That way the glue can penetrate, and give a 'better' bond.. If you seal it first, you are only gluing 'sealer-to-sealer' and that depends whether your concoction of choice has penetrated the MDF, or 'just' sealed the surface..

If the fit is a good as you say (which I would think it is) I would seal afterwards, before attempting an rubbing down.

 

[maxi-model]

Thank you PhilP, that seems logical. I will be probably sticking (quite literally) with PVA, although my preference in the past with ply a wood kits has been CA. I have another 6 Resurgam SAR kits to do as well as this one. 3 are passenger stock. Should keep me busy for the 12 weeks furlough my employer has put me on. Max

 

[PhilP]

If I were using PVA.. I would use an 'external' or 'waterproof' variant, myself..

Personally, the jury is still out on MDF-based kits / stock?? - I really want something for my Garden Railway that can be left out in the garden.

I have a couple of cheap wagon kits.. I intend to leave these outside, as an experiment. - Pretty cruel to leave a container, made of MDF, out in the British climate..
One will have small drain-holes, the other will fill with water, and then either be tipped-out (if running) or left to evaporate naturally..

 

[GAP]

If I were using PVA.. I would use an 'external' or 'waterproof' variant, myself..

Personally, the jury is still out on MDF-based kits / stock?? - I really want something for my Garden Railway that can be left out in the garden.

I have a couple of cheap wagon kits.. I intend to leave these outside, as an experiment. - Pretty cruel to leave a container, made of MDF, out in the British climate..
One will have small drain-holes, the other will fill with water, and then either be tipped-out (if running) or left to evaporate naturally..

I will eagerly await a report on the results of your experiment as I have some scratchbuilt points that sit on a base of MDF and I was wondering if I could leave them outside permanently and what I could seal them with.
I read articles that if I paint MDF to seal it of water does get ,and I am sure it will then it is trapped and the damage starts.
I did soak some plywood in old sump oil and that lasted for about 6 years out in the garden before de laminating (not rotting).

From my experience of making wagons with MDF components it is best to glue first then seal/paint as a better bond will result.

 

[dunnyrail]

Like most wood or cardboard based materials outside life will be limited if left outside all the time, especially if in contact with damp earth.. We used some so called external MDF on the Ruschbahn (against my better judgement) despite additional precautions it was trashed in 3-4 years which I think for MDF was above and beyond the call of duty.

However for rolling stock, normal precautions would suggest if it starts to bucket down driving said wet stock indoors with a quick dry off should see no damage ensue.

Incidentaly I had a friend that built a Viaduct out of Cardboard, coated with Shellac and plonked on some sort of perhaps brick base. Lasted for quite a few years outside 24/7/365. So perhaps a finish coat of Shellac once built before painting may be a worthwhile investment. Note that I have not tried this myself.

 

[maxi-model]

Various subassemblies finished, fettled and a dry fit tested. MDF sealer airbrushed on from the tin, a couple of coats. So next is to get it back to its subassemblies and start painting. Weather forecast is good for the next few days, so a bit of running and a bit of painting and building. Max

 

[Rhinochugger]

I missed Wednesday's question, but like Phil, I would have glued first. For wood (and MDF) I have been converted to Titebond 2 (can't remember the main active chemical) but it is an excellent exterior wood glue, although it takes a while to take, so you have to put the pieces in some sort of clamp while it dries. But when it dries ................... that's it!

The only thing that possibly goes against the 'glue first' principle is when there are going to be areas that you cannot reach to paint or seal after construction or partial construction.

tac has successfully sealed before gluing (I saw his models at Peterborough last year) but my preference is for gluing first - get it rock solid with a truly waterproof glue

 

[Northsider]

I glue first, seal later. If you use a water-based sealer then it is probably PVA anyhow, so I suppose you are gluing like to like. A couple of coats of Halfords primer on top usually makes it good enough for painting, although the lasercut edges don't take paint especially well -I would make sure these have been sealed first.

I have used CA glues on lasercut MDF and you get fairly acrid fumes off it when it hits the charred edges; suffice to say, they probably don't do you any good.
Much of my carriage stock is MDF (some with a 0.8mm ply skin) and it is quite happy outdoors for running sessions, icluding steam and oil from live steam locos. A painted MDF van body goods shed has lasted about six years outdoors (i.e. about 2000 days), which is five years longer than I expected. I'm pretty certain that's unusual, though!