This article shows techniques and provides advice on how to fill gaps and hide seam lines on scale models. Information on the different types of filler available is contained in the article ‘All About Fillers’.
Avoid The Gap
Instead of filling a gap it is almost always better to avoid having the gap in the first place. Although some gaps cannot be avoided there are many that can. Every part should be test fitted before any glue is applied. This has several advantages:
- It shows the modeller where glue needs to be applied;
- It gives the modeller a ‘dress rehersal’ on where and how to hold the pieces in question and whether any support will be needed while the parts set;
- It shows up whether there are likely to be any gaps or steps between the parts.
A step in the level between two parts is just as unsightly as a gap between them and both can often be avoided, or at least reduced. Take a look at where the parts join and try to identify any irregularities, or high points that are causing the gap/step. If these can be sanded, or pared away the parts might fit better.
Another possibility is to use clamps, rubber bands or masking tape to hold the parts tightly in place while the glue dries and this will often improve the fit.
A further way to reduce gaps is to deliberately use excess glue so that it will ooze out between the parts and fill any gap. This is one area where gap-filling superglue is very useful. Caution should be used when doing this because if too much glue is used it can not only fill any gap but might run out on to the model’s surface causing damage to any surface detail. Before using glue to fill a gap bear in mind how much damage might be done if too much glue oozes out and always err on the safe side. It is better to use too little glue and leave a gap rather than too much and ruin surface detail.
Check You Really Need To Hide The Gap/Join
Manufacturers have become very clever with how they break models down into individual parts and ofter arrange for seam lines to occur when there are joins on the real vehicle. This is particularly so with tanks that are often constructed out of large slabs of metal welded together. You may find that on a model there is a seam where on the real tank there is a noticable weld bead.
In this situation, rather than hide the gap you will need to make it look like a weld bead. In other cases, you may not need to do anything at all as the join line on the model may look very like the join on the real thing. This is an area where research photographs are very useful.
Another thing that model manufacturers have become good at is hiding join lines created in the early stages of a model with parts added later on. For example, the addition of storage boxes, fuel drums etc. may completely hide a join or gap. The message here is to check the later stages of construction before spending time hiding a seam or gap.
You should also consider whether the gap could be hidden, or disguised by something you can add to the model. This is not so easy on aircraft, but on military vehicles the application of mud to the wheels, or stowage, tarpauline or camouflage nets to the upper hull could be used to hide an unsightly gap. We are not suggesting that poor modelling should be disguised by the liberal application of mud and stowage. However, if adding weathering and stowage will enhance a model then it can perform a dual role of saving the need to fill gaps.
How To Fill Gaps
Remove Any Difference in Levels
Before filling a gap make sure that there is no difference in height between the two parts on either side of the gap. If there is a step up or down in levels then it should normally be removed by paring it away with a knife or sanding BEFORE any filler is used.
If this is not done then the shape of the model may be distorted and it is surprising how obvious this sometimes appears on a finished model.
The diagrams on the right demonstrate this effect.
The first diagram shows a cross section of an aircraft fuselage where two halves have been joined together. At the point of the cross section there is not only a gap between the two halves but there is a step in height. Step 2 shows the filler (in red) that has been pressed into the gap and on to the surface of the model. Step 3 shows the effect when the filler is sanded away. The fuselage half which stands proud is also partly sanded away with the filler, but even so the result is that there is an uneven transition where one half joins the other. The filler has disguised the gap but has only partly disguised the difference in levels.
This remaining difference in levels can be removed at this stage by further sanding/paring/scraping, but it is made more difficult by the presence of the filler.
In the second lower diagram on the right, the starting situation is the same. However, in this case the difference in levels between the two fuselage halves is removed by paring and sanding away the higher of the two fuselage halves (step 2) before attempting to fill the gap. Clearly, care is needed to minimise any damage to surface detail. The filler is then added (step 3) and when fully set is sanded away as normal (step 4).
The difference with the second method is that there is a smooth transition from one half of the fuselage to the other. You will also need to use and sand less filler because you are only filling a gap and not disguising a difference in levels.
Fill That Gap
Finally, if you cannot prevent a gap and are sure it is not meant to be there and it will be visible on the finished model, there is no alternative but to fill it. There is no magic formula for doing this – it comes down to common sense and practice.
When using model filler, it is necessary to work fairly quickly because it air dries and quickly forms a skin. Use the minimum amount of filler. Taking small amounts smooth it down into the gap avoiding getting it on to the surface.
Any excess will be sanded away so there is no point making it look pretty at this stage, just get it into the gap and move on. Replace the top on the tube as soon as possible. If any filler is smeared where it is not wanted then it should either be wiped off immediately or left to harden fully.
Make sure that the filler is completely set before sanding or scraping away the excess – no harm will be done by leaving it to long, but if it is sanded before it is completely set, a mess will be created since both the filler and some of the plastic will have been softened.
When using epoxy putty there is more time to work and it is usually preferable to get a good finish that will not need any sanding. Smooth the filler into the gap, then scrape or wipe away any excess. The putty can usually be softened and smoothed with a wet finger. If there is any excess to be scraped or sanded away then this should be done when the putty has partially hardened, but before it has set rock hard.
Gap-Filling (thick) Superglue
Gap-filling superglue requires a different technique. It can be applied at the gluing stage so that it fills any gaps as the parts are put together. However, care is needed to avoid putting too much on the parts that will ooze out when the parts are put together.
The other method with gap-filling superglue is to glue the parts as normal and then use a bead of the superglue to fill any gap. A quick spray of accelerator will harden the superglue almost instantly. Any excess can then be pared away with a knife or sanded smooth. The superglue may continue to harden for some time so any sanding should be done as soon as the superglue is hard enough to do so.
Some modellers use superglue together with a filler or binder. Baking soda is often used and I have heard of one modeller using talc. Doing this makes the superglue less likely to crack and more resilient. It also makes it easier to sand. Talc has the advantage that it is very stable and does not degrade. Also the superglue remains sandable even if left for a long time whereas with baking soda the superglue goes rock hard with time. Talc has the disadvantage that it takes longer to cure. When using talc take special care not to breath it into the lungs.
A Practical Example
Alternatives To Sanding
When using most fillers the normal process is to fill any gap and add a little extra filler to be sanded later. This works for most people in most situations. However, there are a couple of alternatives to sanding.
Scraping with a blade
Sometimes scraping over the filler with a knife blade is a better way of removing excess filler. This is particularly useful in confined spaces, corners, or where there is surface detail nearby that might be damageed by sanding. I find that a rounded blade works best and tilting the blade slightly in the direction I am scraping removes the filler without damaging the surface of the model.
Smoothing the filler
Fillers that are based on toluene (see the label on the back of the tube) can be thinned with acetone. Acetone can by bought from most chemists and it is usually the main component of nail polish remover. Using a brush or cotton bud and a little acetone, it is possible to smooth over the filler or wipe it away to leave a flush surface. Only use a little acetone, do not soak the brush/bud in it.
This method is particularly useful if you want the filler to be built up or sculpted for example forming a canvas cover around the base of a gun, or a weld bead where metal railings are fixed to the side of a vehicle.
Further information about using epoxy putty can be found in the article ‘Using Milliput’