Monday, September 26, 2016

Back to Basics 4: Fixing Surface Defects

Plastic injection is a very efficient and highly accurate method of mass-producing model kits. However the process is not perfect, and some physical flaws can still appear on the surface of some parts. The most common flaws we might find are mold lines, flash and sink marks. These surface defects are easy to address and doing so will be well worth the time spent fixing them before painting your model. In this guide, we'll go over each type of defect and discuss a solution.

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Mold Lines and Flash

Mold lines are raised lines or edges that aren't actually part of the kit's intended design. They appear because of small gaps in the metal molds. The molten plastic fills these gaps during the molding process, leaving behind these unsightly lines. Flash is caused by excess plastic leaking out of the mold in a few places. These can typically be observed near and around part edges, and occur more often on thin parts such as the fingers, vents and V-fins.

A good trimming with a hobby knife will usually do the trick for most mold lines and flash. Less prominent ones can be shaved off by holding the blade perpendicular against the surface of the part and lightly dragging it along the mold line several times. Be careful not to let the blade dig into the actual surface of the armor. Clean it off completely with some sandpaper in increasing grits.

Sink Marks

Sink marks appear as little dimples or dents on the surface of a part. They may be caused by molten plastic not filling the mold sufficiently. They can typically be found on the surface of armor where there is supporting frame on the underside of the part. They can also appear over a supposedly fixed seam, possibly due to sagging of the plastic as the cement welds the sides together.

Sink marks will need to be filled in with putty or some other fine filler. Sand over the mark and surrounding area with some lower grit sandpaper, or scrape over it with a knife. This will give the filler something to cling to. Apply filler over the area, making sure there is more than enough material to make a slight "bump" where the sink mark used to be. Allow the filler to properly cure.

Once completely solid, you can then file or sand down the bump to level the surface. Be careful not to sand it down too heavily or you risk removing the filler completely, bringing you back to square one. Work patiently and keep checking if you've already leveled the surface. Polish it up with higher grits to a perfect paint-ready finish.

In summary...

The Step

The most difficult surface defect to deal with is what I like to call the "step". The step occurs when two halves that come together are supposed to make an even surface, but instead end up misaligned. Even after fixing the seam between them, there is an obvious "step" from side A to side B, easily identified by dragging your fingertip or nail across the seam line.

To address this you need to trim down the surface of side B to match side A. Cement the halves together as discussed in the seam fixing guide. Some filler over the seam can also help in smoothing out the transition between both sides, and will ensure the seam does not show through. Once cured, you'll need to work down the higher side with a flat file. If side B is significantly higher than side A, expect to remove a LOT of plastic.

Once the sides are more or less level, switch to sandpaper. If the surface is curved, plain sandpaper can be used. In this example the surface is flat, so I used sandpaper fixed to a solid block to preserve the flatness of the surface. Sanding revealed a few more sunken areas along the seam...

The sink marks were filled with some cement topped with fine plastic shavings collected from the filing process, allowed to cure, then cleaned up with sandpaper to a perfectly smooth finish.

Since the top surface was sanded down significantly, the beveled edge had to be restored. You can read more about this technique HERE.

In this example, there was a surface detail in the way, but it was too tedious trying to work the file around it, so I just added the detail back with some pla-plate once the step fixing was done. The part was washed and given a layer of primer to check for any surface imperfections that might have been missed. 

The image below shows another part where step fixing was applied. Primer revealed that the surface was still slightly uneven, so I had to use putty and re-sand. You can clearly see how the putty (beige) and primer (grey) are helping to even out the surface.

In summary...

If you've encountered these defects before and were able to address them differently, feel free to share your techniques in the comments section below!

That's all I have time for today, until next time, keep building plamo!


  1. hi,
    a budget friendly filler is super glue.
    on the step subject, you may cut the pegs off, align and glue it

    1. Thanks for the suggestions! Never thought about messing with the pegs to get the surfaces to align when adressing the step problem, I'll remember this for succeeding projects!