Preparation before NMF?

[Courageous]

Help required from the experts...

Currently I have a couple of builds that will end with NMF. These builds are undergoing surgery and together with fit problems, will require filling of some sort. I have read somewhere that your normal filler will be no good as this shows up in the NMF models. I know that I can use superglue for small gaps as this can be sanded and 'polished' before the coat(s) go on. What do you guys suggest to my query? As a 'heads-up' I will be using Humbrol Metalcote for my NMF.

TIA

Stuart

[steelpillow]

I always go over filler, dobs of glue, bits of plastikard, etc. with a coat of matt grey to show up any blemishes - light to med-dark according to topcoat. Same for NMF or plain. Fill, sand and repeat until smooth as a baby's proverbial. For a bright NMF, then apply a gloss black undercoat.

[Miggers]

Echoing steelpillow's doctrine,get it as smooth as a baby's bum and squeaky clean.

Nothing shows up dust,hairs,fingerprints,glue smears and any other substrate detrius like NMF.

Get your gloss black shaving mirror glossy and your NMF will looks the dog's doofers.

[Duncan B]

Agree with the above, the filler will not be a problem providing you put down a good primer and make sure everything is smooth.

Duncan B

[pivokrevnik]

You have to use either superglue or any two-part putty (milliput, tamiya polyester, various epoxides etc.)

Any simple, one-part, solvent based putty (Tamiya grey, thick surfacer etc.) will shrink and reveal the seams again. This happens when you least expect it, no matter how long do you let the putty cure (ok perhaps if you wait for months...).

After a period of denial, when I tried different putties, let them cure longer, applied them repeatedly etc., I resorted to superglue. Which is really the best option in most of other aspects as well - hard, rescribes nicely, can be polished to glass-like surface.

I lightly smear ALL seams, because even seemingly perfect seams glued with extrathin (or any solvent glue I guess) will sink a bit over time.

It's really worth the effort, as it prevents nasty surprises and extra work later.

[RJP]

You have to use either superglue or any two-part putty (milliput, tamiya polyester, various epoxides etc.)

Any simple, one-part, solvent based putty (Tamiya grey, thick surfacer etc.) will shrink and reveal the seams again. This happens when you least expect it, no matter how long do you let the putty cure (ok perhaps if you wait for months...).

After a period of denial, when I tried different putties, let them cure longer, applied them repeatedly etc., I resorted to superglue. Which is really the best option in most of other aspects as well - hard, rescribes nicely, can be polished to glass-like surface.

I lightly smear ALL seams, because even seemingly perfect seams glued with extrathin (or any solvent glue I guess) will sink a bit over time.

It's really worth the effort, as it prevents nasty surprises and extra work later.

Let's not confuse filler with adhesive. One is not the other and treating these two very different products as interchangeable will lead inevitably to the problems noted.

A properly constructed joint will be firm and stable before the application of any filler. There are often gaps or unevenness but the joint will not move. Shim it if you have to but let it cure completely. Don't go near it with filler until it has cured.

Filler is used as the name implies, simply to fill the gaps. It can be shaped, filed and sanded but using it as an adhesive is simply asking for trouble. And failing to let it cure right out before proceeding is essential - that is where the shrinking comes from. Murphy's Law being what it is, the cracks and crumbling will develop only when it is too late to fix them.

A normal feature of tube fillers is their porosity. Once it has been shaped and left to cure the surface will not match polystyrene. That is when a primer coat comes in: it will stabilise the surface, fill minor pores and impart a uniform surface.

Primer need not be expensive or particularly technical - I quite happily use good old Humbrol gloss enamel applied with a brush. Once it cures it can be sanded out to a smoothness satisfactory for most finishes though NMF adds quite a challenge.

The key is to understand the materials and use them with due care for their properties.

Edited November 30, 2014 by RJP

[pivokrevnik]

So, the conclusion of your long, semantic, cpt.obvious-like theoretical lecture is, don't use superglue as a filler (because that will be confusing a filler with an adhesive!), but brush on a gloss Humbrol enamel instead (because that is a key understanding of materials and their properties!) Umm...what?

Problem is, that actual curing time is much longer than you'd think, so waiting for "complete curing" has no practical use, unless you build one kit a year. Even after weeks or months a hairline seam could reappear (btw it can actually happen even with superglue, I was told, but I did not experience this so far).

This is not an issue for most finishes, but NMF is unforgiving.

Why take your chances, if you can simply use superglue, which acts as a perfect polishable filler AND a strong adhesive reinforcing the joint?

Btw I don't want this to sound personal, but it's always better to show your own work when you lecture others. Let's say it helps to put the advice in some context of one's own quality standards - what is okay for one can be inacceptable for other.

Edited November 30, 2014 by pivokrevnik

[RJP]

Shrug. Take offence if you wish but do try to take the point.

RJP out.

[Steve Noble]

Preparation would be the same as any painted finish. Fill and sand any join lines until they are perfect, then primer, sand and top coat. For filler I use two part polyester car filler first, followed by primer, then a 1K acrylic car putty to take care of any pin holes. It's never let me down and there's no sinkage

[Duncan B]

OK here are a few pictures to show how I went about filling and then NMF'ing my Lightning F1 conversion. I used various types of filler on this one, the cream stuff is Mr Putty and the green stuff is Games workshop's Liquid Green Stuff, no superglue used at all.

As you can see lots of filler present.

Now all the filler has been sanded back and polished, the first pass of primer revealed some spots that needed correcting so I applied a little more filler and sanded back again as you can see above.

And finally the end result, can you see any shrinkage or joins? I hope that helps and shows that you don't have to use a 2 pack epoxy or superglue (but there is nothing wrong with using them if you want to).

Duncan B