Friday, 6 January 2012

Cutting, Sanding and Putty related Questions

1. What is a 'Cut' and 'Trim'?
It is just a term we use to describe for the way we cut parts off a runner. It means we cut the parts off the runner with a little allowance, leaving behind a little nub on the part. We then trim of the nub down with the side cutter (Place the flat edge of the side cutter flush with the part to trim).

2. Why should I bother to use the 'Cut' and 'Trim'?
It is not necessary to do it in two steps. We recommend this method, because some parts are attached to the runner in a awkward position, which does not allow you to get a clean cut. Sometimes, you may overcut into the part and leaving behind a nasty hole.

Question 3 - 5 applies mainly to modelers who do not intend to paint their models.

3. I used a good side cutter, when I trim the nub it still leaves behind a white mark. How can I prevent that?
Most people would experience this, especially when cutting darker colour plastic which are usually made from ABS plastic. The white mark is a stress mark caused by the bending of plastic as you cut. You can reduce the stress on the plastic, you can try trimming the edges of the nub, instead of cutting across it. Smaller cuts reduces the stress on the plastic and remove the remaining nub with a sharp modeling knife.

4. I tried the method you describe in step 3, but there are still stress marks?
There are several ways to remove the stress mark. Firstly, you can try touching up the stress mark by using a Gundam marker of the same or similar color. Secondly, you can try to melt the plastic with Tamiya Extra Thin Cement (for Polystyrene PS parts)  or Mr Cement S ( for ABS parts). As the plastic melts, it returns to its original colour. Leave the plastic to harden for a day before sanding. Exercise caution when apply cement as it cause your plastic to melt, excessive cement could cause undesirable effects.

5. I used the cement to remove the stress mark, but when I sand, I discovered that it made a hole. I do not intend to paint, so I can't fill the hole with putty. What can I do?
You can try shaving some plastic off the runner with the same colour as the part. Place the shaving in the hole and melt it with cement. Repeat the procedure till the hole is completely filled with a little excess. Wait for the plastic to harden before sanding.

6. How do I know if the plastic has harden?
When you are sanding and the plastic coming off is sticky, it means that the plastic has not completely harden. When you sand, it should be dry and powdery.

Sanding Updated on 7/1/2012
1. Why kind of sand paper should I use for sanding my Gundam model kit?
Hobby Sandpapers are always recommended, because the are designed and tested for plastic. If you still wish to use commercial sandpapers, please be careful in selecting the right types for plastic. Using the wrong kind of sandpaper can cause deep scratches on your plastic parts which are quite difficult to remove.

2. What are the numbers printed on the sandpaper?
They represent the grit of the sandpapers. There are many grit of sandpapers, ranging from 180 to 2000 or beyond. The bigger the number, the finer the sandpaper.

3. What grit of sandpaper should I use?
Generally sanding of plastic is done with sandpapers between 400 and 1000 Grits. We start with 400 grit to remove the nub and move up to 600 grit to reduce the scratches 400 grit and finish with 1000 grit.

4. So what are the other grits used for?
The lower grits like 180-320 are used to sand harder materials like superglue or epoxy putty. Higher grits like 1200-2000 are use to achieve super fine finishes (especially for gloss finishes, like car model kits)

5. I am using 400 grit hobby sandpapers to sand my model kit. Why is it I am still getting deep scratches?
Tip for preventing deep scratches from forming, use less force when sanding especially when using a new piece of sandpaper. Sand lightly over a larger surface, do not focus only on the nub. Sand with a circular motion.

6. Why is it that the part I am sanding is sticky?
Generally, it means that your cemented parts or puttied parts have not completely cured. Please try to sand in another couple of hours.

7. Should I use a file, it's definitely faster than sandpaper?
It is personal preference, unless you can control the use of a file, you will tend to leave behind very deep scratches. Only use files when you need to remove super glue or epoxy putty quickly and probably have to follow up with 180 - 320 grit to remove the deep scratches.

FAQ to be continued...


  1. Regarding Sanding, is it nesscary to do a Light Sanding before spraying a Gloss Paint>Gloss Coat? does the Light Sanding affect the final product?

  2. Hi Slyvas, it isn't necessary to do any light sanding before spray gloss coat, just make sure that the surface is clean. Normally we would sand to remove any sink mark (little indentations in the plastic formed during the molding process) present in the plastic to smoothen the surface. If you do decide to sand, it would be advisable to finish with 1500-2000 sandpaper before applying gloss coat.

  3. Alright. Thanks For Your Reply