Preparing For Paint Part II: Filler Materials

From sink marks, to sunken ejector pins, to seams fillers assist the builder in making two parts look like one or a rough surface look smooth or can manipulate two parts to look like one but which one do you choose out of the array of tubes and bottles on a hobby store shelf? The following list with pros and cons of each filler type along with their suggested uses should help shed some light on the subject.

Putties

Pros:

• Easily available
• Ready to use
• Easily Sanded and shaped

Cons:

• Can shrink or crack
• May dissolve plastic if used in heavy amounts
• May dry too quickly to be workable depending on brand
• Toxic fumes

Putty is the most common of fillers. Generally found in a toothpaste style tube, each has it's own advantages and disadvantages. By far the best I have found is Bondo spot & glazing puttor 3M brand spot & glazing putty. Unlike hobby marketed putties, Bondo is made for auto body sheet metal work so it will adhere to metal and resin where the hobby specific putties will only adhere to styrene. These putties contain a solvent that mildly dissolves the surface in order to bond to it so care should be taken to not apply them to heavily. In addition to possibly marring the surface, large amounts of putty may shrink or crack creating more work for you in the end. If you need to build up a large area it's better to apply the putty in thin layers rather than one heavy one. When wet putty is easily formed over molded detail such as uniforms where limbs attach on scale figures and when dry can be easily sanded and carved. The only major disadvantage I've found is some brands, namely 3M Acryl Green can dry too quickly rendering them unworkable before they even leave your palate and make it on to the model. If necessary Squadron putty can be thinned with Testors liquid cement which will extend it;s drying time slightly and allow it to be brushed on to more compact areas or even forced through a hypodermic needle to form weld beads or other details.

Recommended Brands

• Bondo Spot & Glazing
• 3M Spot & Glazing
• 3M Acryl White
• Squadron Green Putty

Not Recommended

• Testors putty for plastic models  (Does not adhere, flakes off.)
• 3M Acryl Green (Dries too quickly)
• Bondo 2 Part Body Filler (in the can) (Hazardous fumes, too potent for styrene)

Epoxy Putty

Pros:

• Long drying time leaves plenty of time to sculpt or mold them
• Bonds well
• Clay like texture makes it ideal for large applications and sculpting

Cons:

• Not ready to use, must be mixed
• Long drying time may put a project on hold for a few days dependent on weather
• difficult to sand when dry

Like two part epoxy glue, epoxy putty comes packaged in two parts that must be thoroughly mixed before it will dry. Milliput is by far the most well known of these two part putties and comes as two cylinders of a somewhat hard material that becomes softer and more workable as it is warmed and mixed. Epoxy putties work wonders for filling large gaps since they do not shrink or crack and have a relatively long drying time leaving them workable for up to several days depending on temperature. When I use the term "depending on temperature" in basic terms I mean the hotter the temperature the faster epoxy putty dries. In my shop for example an application of epoxy putty will dry in 24 hours to the point where it can be sanded whereas in the winter when the temperature in my garage hovers around the mid 40's to low 50's it may take up to 72 hours or more. With these putties it is necessary to sculpt and shape them as much as possible before they dry completely as once they achieve rock hard status they can become difficult but not impossible to sand.

Recommended Brands

• Milliput
• Apoxie Sculpt
• Tamiya Epoxy Putty

Gap Filling Superglue

Pros:

• Ready to use
• Instantly dry with accelerator 

Cons:

• Very unforgiving
• Difficult to sand and cannot be shaped

Superglue, the same stuff you use to join parts together can also be used as a quick gap filler. The stuff you're looking for is the thicker gel type superglue which holds it's form and doesn't want to spread around too much which could mar nearby details. When combined with an instant set "kicker" the filler dries in seconds and is ready to be sanded. I generally only use gap filling glue for small applications on flat surfaces or butt joints or to quickly fill an ejector pin mark or small sink hole when preparing parts for assembly.

Recommended Brands

• Zap A Gap
• Kwik Bond Gel

Gunze Mr. Surfacer

Pros:

• Brushable Filler
• Quick Drying
• Excess can be wiped away with thinner on a Q-Tip
• Easily sanded

Cons:

• Harsh Fumes
• Can only be used to fill shallow defects

A friend of mine turned me on to this stuff about two years ago and I love it! Mr Surfacer is basically a very thick paint that comes in various grades from coarse to super fine. I like to use Mr Surfacer as a post fill material. What I mean by "post fill" is for certain seams after I have used the other options listed above I will apply a thin coat of Mr Surfacer 1000 to fill in any remaining small defects. After application excess can be wiped away with a cotton swab dampened in lacquer thinner or sanded gingerly with 800 or 1000 grit sandpaper. Mr Surfacer can also be thinned and applied through an airbrush and is also available in spray cans which can make the 1200 grade an ideal primer or sealer coat prior to primer.

Recommended Brands

• Mr Surfacer 500
• Mr Surfacer 1000
• Mr Surfacer 1200

Part three of this series will cover the sanders, scrapers, and files needed to tone down a rough surface and shape fillers to a perfect contour, stay tuned!

<<<Part I                                                                                                                                 Part III >>>