How to Glue Parts Together

Introduction

Liquid Polystyrene Cement – one of the most widely used glues for plastic model kits.

This article covers the most fundamental of all scale modelling skills – gluing parts together. It follows on from the article ‘All About Glue’ that describes the different types of adhesives commonly used in scale modelling.

There are so many variables with this subject that it is difficult to know where to start.  Model kit parts come in such a wide variety of shapes, sizes and materials and there are so many adhesives and applicators on the market, it is not possible to condense the technique into a few simple rules that will apply to all situations.  Perhaps that is part of the reason that every modelling project provides a unique challenge.

I have chosen to approach the subject by suggesting a few principles that should be followed most of the time and then giving step by step examples of different situations.

Basic Principles

Principle 1: Clean Up Parts

Removing seams and molding flaws

If you have not already done so, this is the last chance to check and clean up the parts you are about to glue.  Remove any seam lines, sink holes, ejector pin marks and flash.  If you want to enhance any details this is probably the best time to do it because once it is on the model it will be more difficult to make alterations to the part.  If you have pre-painted the part then make sure that the area that is to be glued is free of paint to ensure a good bond.

Principle 2: Always Do A Dry Run

It is always best to do a dress reherasal of fixing the parts in place before applying any glue.  Even if you are fixing several parts which appear all the same check that each one fits first.  This can seem to be tiresome for parts that are repeated such as fitting the wheels on a tank.  However, just because the first nine wheels fit on their axles perfectly, it does not mean that the tenth will.  The reasons for these dry runs are as follows:

1. To check the parts fit well

This is the last chance to identify any changes that will improve the fit of the parts and reduce any gaps.  It also identifies whether the parts will need to be clamped or held together while the glue sets, so that the appropriate clamps, rubber bands, masking tape etc can be at the ready.

2. To identify where to apply the glue

You will see where the parts meet and therefore where glue needs to be applied and whether one or both parts need to have glue applied.  Depending on how well the parts fit, your choice of type and viscosity of glue may change.

How much glue to use and where to apply it is critical but it is normally best to err on the side of too little glue.  When putting a bead of glue along an edge to be glued hold the glue dispenser at a 45 degree angle and put the bead on the interior corner of the edge so that it is unlikely to ooze out of the seam (see diagram 1 on the right).

3. To work out how to hold the parts

If the parts are small, or they need to be placed into awkward places, you need to work out beforehand where you will hold them and how you will manouver them bearing in mind that one or both parts will have glue on them that will prevent them being touched in places.

Principle 3: Choose Your Glue

There is no one glue that is appropriate in all places.  When gluing two styrene parts then either poly cement, cyano or epoxy glues can be used.  When gluing resin or metal parts then poly cement can not be used, but that still leaves epoxy cement and thin, medium or thick cyano.

Your choice of glue will depend on the size of the parts, how well they fit, how long you need to work with the part to get it in the right position, how strong the bond needs to be and whether or not it matters that any excess glue is visible.

Principle 4: Make sure that you have the necessary supports

These two halves fit very badly and had to be held tightly together with rubber bands and maksing tape whilst the glue set.

The dry run will identify whether the bond is instantly self-supporting or whether it needs to be held in place while the glue sets.  Depending on how you intend to support the bond do the following:

• Check you have the right number and size of rubber bands
• Check you have the right size clamps and they are unscrewed
• Cut sufficient strips of masking tape of the right length and width

Having established these principles, we can move on to some practical examples:

Examples of Fixing Parts In Place

1. Small part into a locating hole

This is where the part is to be fixed.  A locating hole at the rear of a 1/72nd scale Abrams tank	This is the part to be fixed – a tow hook.  The locating pin is indicated by the red pointer.  The part is so small it is held in self-grip tweezers throughout.
The part was test fitted and found to be a loose fit so it was decided to use cyano glue to give a quick bond.  A single drop of cyano was put into an alu-foil ‘cushion’.	A small brass rod was used to pick up a tiny amount of the cyano glue
The cyano glue was then transferred to the locating hole on the tank’s rear	The tow hook was then put in place and held fast for a few seconds until the cyano took grip when the tweezers were released
This is the final result.  The tow hook securely fixed with no excess glue.

2. Fixing an unstable part

The part is a return roller attached to it’s axle which fits into a shallow locating hole in the side of the vehicle hull. The problem is that the locating hold is so shallow that it does not hold the part firmly.	Since there is nothing to make the part stay in the correct position medium poly cement is chosen as the glue because this will give time to check the position of the part before the glue sets.  A small drop of poly cement is applied directly to the end of the axle.
Here the drop of poly cement can be seen on the end of the axle.  Because the part only fits loosely in the locating hole there is room for this amount of cement without it oozing out	The part is pressed into the locating hole and left for a few seconds for a bond to form.  When the tweezers are released the part is still unstable but there is enough of a bond to hold it in place
Here is the part fixed in place.  Using a corner of paper a check is made that the axle is perpendicular to the hull side.  Since the part is still unstable it is easy to move it at this stage.	I set the timer on my watch for 5 minutes as a reminder to check the alignment of the part to make sure that it was still perpendicular to the hull side.  By this time the bond was sufficiently strong that it could be safely left without fear of it moving.  However, until the glue is completely set the hull needs to be left alone.  For this reason, some modellers may have chosen superglue as it sets quickly allowing the modeller to carry on with the hull construction.  The problem with superglue is that it may not give enough time to align the part before it forms an immovable bond.

3. Fitting a hatch

Here is the part to be fixed.  The driver’s hatch on a 1/72nd scale Abrams tank.	The part fits into this triangular hole which has a recessed lip around the entire circumference.
The hatch fits perfectly into place and as there is access to it from behind it was decided to glue it in place.	Note how the index finger of the left hand holds the part firmly in place whilst liguid poly cement it brushed around the circumference.  Capillary action draws the cement between the two parts.
This close up shows where the glue has been applied.  Care is needed not to apply to much glue because it could run out the other side and ruin the top surface of the model.  Although liquid poly cement was used in this example, superthin cyano adhesive would have worked just as well.	This is the part fixed in place.  Even if it was not possible to get behind the part this method could be used by using a tiny brush to apply the liquid cement which would be drawn between the parts by capillary action.  However, great care would be needed to avoid getting any glue on the top surface of the model.

4. Joining two halves together

In this example, the two halves of a tank turret are joined.  However, the same method could be used for joining any two halves together such as the two halves of an aircraft fuselage.

Here are the two halves of the turret.	Examination of the top half shows that there are locating ridges but they do not run all around the turret
A test fitting reveals that the two halves fit together well although there are areas where a gap is present and the two edges are not level.  However, it is decided that these are best corrected after gluing.	The test fit also revealed that there was one area on the lower hull that should not have adhesive applied. This area was marked with a pencil.
A bead of medium viscosity poly cement was run around the edge of the lower hull except for the area marked with the pencil.  Since the join line is underneath the hull and not visible it did not matter if any glue squeezed out when the parts were brought together.	The two parts were brought together and then strips of masking tape were used to hold them in place and minimise any gaps.