Gap filling - before or after first primer coat?

[sibomots]

I did a cursory search for "filling" and "gap filling" and didn't find good answers.. If I searched incorrectly, I apologize.

 

Scenario:   A Space Marine-like troop unit.  Torso clam-shell assembly, legs, armed in both hands.

 

Problem:  I've tried it both ways -- either priming the first coat on the whole model and then going back with a filling compound (fill, scrap, sand, repeat until smooth and filled);  Or, after glue together assembly, before primer coat, attempt the fill/scrape/sand/smooth/repeat process.

 

Hypothesis:

I suspect it's better to do the gap-fill after the first coat of primer:  1, it gives a good surface for the adhesive gap-filler to stick and  2,  after priming I'll see where the burrs and mold-lines and mold gaps are (the paint will just enhance the imperfections, so to speak).  But that's just a hypothesis.  I'd like to hear opinions?

 

Materials:

 

As far as the material to use for gap-filling, I'm struggling to find a good material.

 

Is "green stuff" just the best we got?    (either the manual-squeezed-in-fingers kind or the Citadel-type "GS in a jar")

 

I've also looked at:

Humbrol Model filler

Vallejo "Plastic Putty"

 

Has anyone tried Bondo ? 

Are there other compounds that give good adhesion, fine grain for small cracks/seams and is easily sanded/scraped ?  (resists gumming, dries like "plaster" is what I'm looking for.)

 

Thanks -- any insights welcome.

 

 

[Inquisitor Eisenhorn]

Yeah I gap fill after the first primer coat--sometimes the only primer coat and I just use a brush to touch up after gap-filling. 

 

My main gap filler after experimenting with a ton of materials is just plain old milliput, but I dilute it with isopropyl alcohol.  Works great--better if you have some rubber sculpting tools to push/smooth it into the gaps.

 

Marcus Frisoni has a great tutorial on this method on his youtube page. 

[sibomots]

The Milliput+Isopropyl is the exact thing I need.  I had no idea it could be thinned.  Thanks.

 

And, thanks for the comment about primer then filler.  I think that's what I will stick with.  Pun intended.

[Inquisitor Eisenhorn]

The Milliput+Isopropyl is the exact thing I need.  I had no idea it could be thinned.  Thanks.

 

And, thanks for the comment about primer then filler.  I think that's what I will stick with.  Pun intended.

It's not widely known, or at least I've only ever seen Frisoni show this tutorial.  It's been a godsend. 

[Bloody Legionnaire]

Honestly, from a model hobby perspective, both is probably the right answer.

 

However for the sake of the argument at hand, it depends on your primer and the filling agent being used. You ideally want a sandable primer. I prime through an airbrush. The stynylrez primer I have been using is absolutely sandable.. I would not recommend using Vallejo surface primer if any sanding is necessary, however. 
 

I can’t think of a gap filler though that I’d prefer to use after priming. Tamiya gap filler can be thinned and smoothed over a surface with liquid plastic glue. While the intent is re-priming, I know there’s a few primers that may react poorly with liquid plastic glue. Green stuff and milliput on the other hand can be smoothed over fairly easily with water and may lend itself better to being applied after priming,

 

Personally, I think the topic could be presented differently. The question is more “when” should you fill gaps? Fill those that you can see before priming, then after priming, inspect the surface and ensure there’s no imperfections, then correct any you find. The process is more involved after priming and sanding to get back to a good finish before painting.

[Brother Carpenter]

I recently came about greenstufworld.com and they have this very handy comparison table when you select any kind of filler. It's under the more info section, and you can actually download ist as a pdf (maybe something to make a sticky for??)

 

That's how I learned this weekend how to dilute te different kinds of putty's. 

 

https://www.greenstuffworld.com/en/green-stuff/858-green-stuff-tape-6-inches-with-gap.html

 

Check the above link to see what I try to explain above   

Edited December 27, 2021 by Brother Carpenter

[Bung]

Mr. Hobby Surfacer can be used before and after priming for small gaps.

[sibomots]

Honestly, from a model hobby perspective, both is probably the right answer.

 

[ ... ]

 

Personally, I think the topic could be presented differently. The question is more “when” should you fill gaps? Fill those that you can see before priming, then after priming, inspect the surface and ensure there’s no imperfections, then correct any you find. The process is more involved after priming and sanding to get back to a good finish before painting.

 

That is a good point.

 

"When" is the appropriate question.   I should have framed it that way.    This is plastic and the amount of slippery "mold scale" on the surface (I cannot remember the term GW calls it) is also very minor but enough to think about when to gap-fill (before or after the primer aka first paint layer).  My concern was mainly that the slipperiness of the pre-primed surface would not hold the gap-fill compound well.  Hence the question of when to prime (before or after the gap-fill).

 

Many good answers here, and good refinement of the issue.  I appreciate it.   It's been 6 years since I picked up a model and so the memory has faded.

[Arkhanist]

 

Honestly, from a model hobby perspective, both is probably the right answer.

 

[ ... ]

 

Personally, I think the topic could be presented differently. The question is more “when” should you fill gaps? Fill those that you can see before priming, then after priming, inspect the surface and ensure there’s no imperfections, then correct any you find. The process is more involved after priming and sanding to get back to a good finish before painting.

 

That is a good point.

 

"When" is the appropriate question.   I should have framed it that way.    This is plastic and the amount of slippery "mold scale" on the surface (I cannot remember the term GW calls it) is also very minor but enough to think about when to gap-fill (before or after the primer aka first paint layer).  My concern was mainly that the slipperiness of the pre-primed surface would not hold the gap-fill compound well.  Hence the question of when to prime (before or after the gap-fill).

 

Many good answers here, and good refinement of the issue.  I appreciate it.   It's been 6 years since I picked up a model and so the memory has faded.

 

 

I find the best results to be a hybrid approach.

 

I use a sharp hobby knife and micro-mesh sanding sticks immediately on each part after clipping off the sprue, because I know where the contact points were, so I can remove any excess nubs and smooth off. The micromesh sticks come as multiple grades, but I generally only need the first two, with the third for very flat surfaces.

 

I use tamiya extra thin plastic glue, I put this on pegs, and mainly run along the seams to put the pieces together. This does soften the plastic a little, so you can squeeze the pieces together and hold and the pieces will 'melt' together, filling gaps. I do a final pass with the sanding sticks once the glue is dry.

 

At this point, I rinse the models in the sink to remove any left over mold release, and dust from sanding.

 

Since I'm usually batch assembling, If there's any visible gaps in the group, I fill with vallejo plastic putty, or milliput slurry* depending if  let dry, and sand down if needed. I then prime.

 

Depending upon how good a job I've done on the previous steps, everything may be good, or I may have missed something (as you say, the priming does reveal sins better), so it's do the gap filling again, sand down, and re-prime those areas.

 

I prefer to do as much fix up prior to priming, as I find I get a better finish when I do it that way, rather than post-prime and re-prime. Good primer (I use stynylrez) will also help cover minor imperfections. Stynlyrez can be sanded down fine, but I still sometimes get very small 'bumps' when I go from a primed area to now-unprimed, which then show up after the next primer layer, which irritate me. I can sometimes save this with the finest grades of micro-mesh, but it does take more steps.

 

If a model is noticeable covered with mold release, you can rinse in warm water with washing up liquid (dish soap) and a soft toothbrush before doing gap filling; normally a solid rinse under the tap is enough though.

[Xenith]

I did a cursory search for "filling" and "gap filling" and didn't find good answers.. If I searched incorrectly, I apologize.

 

Scenario:   A Space Marine-like troop unit.  Torso clam-shell assembly, legs, armed in both hands.

 

 

I'c curious as to why you need gap filler for these types of models - mine go together seamlessly. Maybe changing the way the model is prepped might eliminate the gap entirely without the need for filler?

 

Otherwise, I always fill before I prime. I want as few layers of paint on there as possible. 

I should have mentioned, if there are any seams in plastic models...I just use my fine poly cement nozzle and put more glue in the join. This melts and fuses it together, quick scrape over with a knife when dry usually elminates visible joins like this. 

[sibomots]

 

I did a cursory search for "filling" and "gap filling" and didn't find good answers.. If I searched incorrectly, I apologize.

 

Scenario:   A Space Marine-like troop unit.  Torso clam-shell assembly, legs, armed in both hands.

 

 

I'c curious as to why you need gap filler for these types of models - mine go together seamlessly. Maybe changing the way the model is prepped might eliminate the gap entirely without the need for filler?

 

Otherwise, I always fill before I prime. I want as few layers of paint on there as possible. 

I should have mentioned, if there are any seams in plastic models...I just use my fine poly cement nozzle and put more glue in the join. This melts and fuses it together, quick scrape over with a knife when dry usually elminates visible joins like this. 

 

 

That's a good question.   I'll elaborate further in case.

 

About 90% of the time, the seams between edges line up and a very small amount of deburring is needed to flush the edges.    If the edges don't contour together cleanly I ignore it when the edge is going to be hidden behind other things.   When the edges cannot flush together and the edge is visible the challenge is adding material to the seam to literally fill a void.

 

A good example I find is in marrying the torso front/back of a Black Templar  troop model.  Black Templar front torso (from the BT Chapter Upgrade kit) with a nominal Space Marine torso back.  The pins/dimple don't match so I need to fabricate a match.  [ the SM back has pins expecting dimples to mate them from the front..  But the BT upgrade kit torso fronts use pins too...  So, out comes the knife...]    After joining, then the belt is discontinuous around the waist -- either because of alignment OR more often because the molds made in the BT Upgrade kit were defective in the sprue.  The belt has a 0.1 - 0.3mm gap in the plastic.  Not a edge alignment issue, just missing material.    Literally a gap on the sides where the belt is.   I could ignore it, paint over it, and the 'gap' be ignored.  I either end up ignoring it, let the gap exist, and paint the model.  Or, I strive to create a smooth continuous surface where it was intended, thus need the gap-fill.

 

So, in general, it's a rare occurrence but it happens and my intent is to create a smooth continuous surface where it was intended.

 

Do I really need to fill it? Not really.

Do I want to fill it?  Yes.

 

Removing the "mill scale" from the plastic prior to priming is what I usually do now (soak in some lukewarm water, scrub with a toothbrush and mild liquid soap, and dry over night).

After the priming layer (usually aerosol acrylic) reveals all the truths about where the real edges and imperfections are.  As the primer dries, and shrinks over the plastic, it will show exactly where the smoothness is (and isn't).  I'll re-examine the model after priming to find those discontinuities and improve the edges.

 

As far as using the glue as agent to fill the gap, I tend to avoid that if possible -- especially for the glue that chemically 'melts' the plastic.  With my fumbling fingers I wind up accidentally getting it on my fingers and then as I press the model (unaware a tiny fleck of the glue is on my finger)  glue from the finger gets onto the flat surfaces not intended -- and puts a melted fingerprint into that surface -- which I have to clean up before priming.   In general, I am very reluctant to use chemically-melting plastic glue, and instead use a formula that bonds w/o the melt.   Granted not as solid, but solid enough for me.  

 

The downside for the extra-glue approach is that I cannot see as well the interface between the extra glue and the as-is plastic -- and end up taking too much material off to get the surface continuous.    Others might (and likely) have great vision to see it.   I understand the approach you describe.

 

For the need of gap-fill, the isopropyl alcohol + Milliput trick has been interesting and productive.   Edges once considered a 'gap too far' to patch and smooth are now after priming just invisible. 

 

My hands are getting to the point where the involuntary jittery shakes are limiting my painting quality, but I'm still trying.  If building models is all I can do these days (haven't had a chance to play the game in 6 years), I'll do what I can to make the model the best it can be before I paint.  My goal is to achieve a primed surface without indication of a discontinuous "edge".

 

Anyway -- that's the rationale.   I'm eager to learn of better approaches.  Thanks for the comment.