How Much Resin Do You Need? The Complete Guide

The question ResinSJ hears most often is “how much resin does a piece this size take?” It is not hard to answer, but getting it wrong costs money and time — mix too little and the piece is short, mix too much and the excess is wasted. This guide covers the calculation method and mix ratios for every type of resin work, and pairs directly with the calculator above.

How Resin Quantity Is Calculated

There are only three steps.

1. Find the volume of the piece from its length, width and thickness.
2. Convert volume to weight by multiplying by the density of the resin. Mixed epoxy is about 1.15, so 1 litre weighs roughly 1.15 kg.
3. Add a waste allowance for the resin left behind in the cup and on the stirrer.

Volume formulas by shape

For an irregular shape that fits none of these, estimate the rectangular box that would just enclose the piece, then deduct the obvious empty space by eye. This gives a usable, safe figure and beats guessing.

How much should you allow for waste?

Resin always clings to the cup wall and the stirrer, no matter how carefully you scrape.

• First attempt, or small pieces — allow around 20%, since the amount left behind is large relative to the total.
• Large pieces — 10% is enough, depending on how well you scrape the mixing container.

Estimating from a previous piece

If you have made something similar before, the most accurate method is to weigh the old piece and use that as your reference. It beats the formula because it already includes the real losses of your own process.

Which Resin Glass Grade Should You Choose?

Each ResinSJ epoxy grade is built for a different job. Matching the grade to the work gives better results and costs less.

The short version

• Want a clear, bubble-free piece that is not very thick — choose 011+.
• Doing a deep pour or a river table — choose 021+ or 021C.
• Doing surface coating or general work where bubbles do not matter — choose 011.

Epoxy Mix Ratios

The product code tells you the ratio. Grades beginning with 011 mix 1 part A to 1 part B. Grades beginning with 021 mix 2 parts A to 1 part B.

The single most important rule: weigh on a digital scale, never measure by volume. Part A is denser than Part B, so measuring with cups gives the wrong proportion. Do not deviate more than ±10% from the stated ratio either — it affects strength, hardness and whether the resin cures completely.

The most common mistake is with the 021 grades, where customers reverse the ratio to 1 part A and 2 parts B. The result is an incomplete cure: a piece that stays soft or tacky. Check the ratio before every mix.

Stirring technique

• Stir continuously in one direction — clockwise only, for example.
• Do not lift the stick in and out or beat the mix like eggs; that drags air into the resin.
• Scrape the sides and the bottom of the cup thoroughly. This is the number one cause of patchy curing.
• Stir for about 3 minutes. Once fully blended the cloudy streaks disappear. If scraping the wall still reveals a clear film, keep going.

Polyester Resin, Cobalt and Catalyst

Unlike epoxy, polyester resin needs both cobalt and MEKP catalyst before it will cure.

About 1 gram of liquid equals 20 drops, which is useful when you do not have a fine scale. Increase the catalyst only once you are confident you can finish the work before the resin gels, or when heavy additive loading is slowing the cure.

Two exceptions: flexible resin uses cobalt at double rate, 0.2%, and pre-promoted clear resin needs no cobalt at all, because clear cobalt is already blended in at the factory.

The safe mixing order

Never combine cobalt and catalyst directly. Doing so generates extreme heat and can produce smoke or fire. The correct order is:

1. Stir the cobalt into the resin until fully blended.
2. Only then add the catalyst.

One more thing worth knowing: once cobalt is added, shelf life drops to 1–4 weeks depending on storage temperature. That is why ResinSJ always asks whether you want the cobalt blended in for you, free of charge.

Fixing the tacky surface

Cured polyester leaves a sticky top surface because oxygen interferes with the cure. You can wash and sand it off, or add Monowax at 0.5–2% to the final layer. The paraffin wax rises and forms a film that seals out oxygen, leaving a glossy, non-tacky surface with better water resistance and no white bloom when wet.

Mould Making

Mould calculations differ from casting because the master volume has to be deducted:

Silicone required = container volume − master volume

If the master is too complex to calculate, submerge it in water and measure the overflow in millilitres. That figure is the master volume exactly.

The critical factor with alginate is time. One bag holds 450 g and mixes with 900 ml of water. You must mix as fast as possible within 2 minutes, and if you reach that point still not fully blended, pour anyway rather than let it set in the cup.

For silicone, if the master is very porous, apply petroleum jelly and wipe it back to a thin film before pouring. Anchor the master to the bottom of the container as well, or it will float when the silicone goes in.

Fibreglass FRP, Step by Step

Polyester resin is the recommended choice for fibreglass work: good value, fast curing, and it wets out glass fibre well. The process has three clear stages.

Stage 1 — Primer coat

Improves adhesion to the existing surface and fills small pores. Mix polyester resin with 5–10% Monostyrene to thin it for penetration, plus 0.5–2% catalyst.

Stage 2 — Laminate

Lay the glass cloth and wet it out with resin, then use a steel roller to drive out air and push resin through the fibres.

• Use roughly 2–2.5 times the weight of the glass in resin.
• Typical work uses three layers of 450 g/m² cloth.
• Mix one layer at a time. A single batch for the whole job will gel before you finish.

Stage 3 — Top coat

The final sealing layer, applied about 1 mm thick with 0.5% Monowax so the surface cures fully, stays non-tacky and finishes glossy without sanding. Remember that Monowax belongs in the top coat only — in lower layers it will prevent the next layer from bonding.

Keep acetone on hand to clean brushes and rollers immediately after use. Once the resin cures on them, they cannot be recovered.

Self Levelling Resin Floors

For coating and floor work there is a simple rule of thumb: 1 kilogram of resin covers about 1 square metre at 1 millimetre thick.

The recommended thickness for a self levelling floor is 3 millimetres so the resin can level itself, and never below 2 millimetres — too little resin pulls itself together as it cures and leaves splits or hollows on the surface, since resin shrinks slightly before it sets.

Common Problems and Fixes

Patchy curing with sticky spots

The cause is incomplete mixing, usually at the cup wall and bottom. That unmixed resin gets poured into the piece along with everything else. There are two fixes.

1. Clean and repair. Wipe the uncured resin away with acetone, let it dry, sand the surface to create a key, then mix a fresh batch and pour again. The new resin flows in and restores a smooth glossy face, though a faint layer line may be visible from the side.
2. Pour over the top. Suitable when clarity and structural strength are not critical, since uncured resin may remain inside the piece.

Soft, sunken or incompletely cured pieces

Almost always a ratio error, particularly the 021 grades mixed 1:2 instead of 2:1. Check the ratio and use an accurate scale every time.

Bubbles in the piece

Bubbles can enter at any stage — transport, stirring, pigment mixing, pouring. By nature they always rise, and they have roughly 30 minutes before the resin gels. The faster you get the resin into the piece, the more time bubbles have to escape.

Resin curing faster than expected

Factors that shorten gel time:

• Batch size. A 2 kg batch may start to harden in 20 minutes, while 1 kg takes around 30.
• Metallic and pearl powders. Gold, silver and copper powders accelerate the exotherm; gel time can drop to about 15 minutes.
• Room temperature. The warmer it is, the faster it sets.

Conversely, on rainy or humid days — especially with polyester — cure time can double, and humidity may also leave the surface cloudy or blotchy.

Working Time and Demoulding

• Working time — about 30 minutes before the resin begins to gel.
• Next layer — epoxy can be poured over after at least 2 hours.
• Demould — after at least 4 hours, taking care to avoid fingerprints and nail marks.
• Full cure — within 24 hours. After that a fingernail will not mark the surface.

Extra Techniques From the Workshop

Getting the same colour every time

Blend the pigment into Part A or Part B before combining the two. This keeps the shade consistent between batches, which matters most for tinted transparent work and multi-layer castings.

When to use Monostyrene

Monostyrene thins polyester resin so it penetrates grooves and wets out cloth. Use no more than 20%, and typically 5–10% on the first layer over kevlar or glass. Bear in mind that it evaporates as the resin cures, which can cause shrinkage and reduce final strength, so use no more than the job needs.

Frequently Asked Questions

How much does 1 litre of resin weigh?

Mixed epoxy has a density of about 1.15, roughly 15% heavier than water, so 1 litre weighs about 1.15 kg.

Why weigh instead of measuring by volume?

Because Part A is denser than Part B. Measuring with cups gives a proportion different from the specification, and the resin will not cure properly.

Can I mix large batches?

The larger the batch, the stronger the exotherm and the faster the cure. Resin Glass 021+ handles about 5 kg per batch without cracking; beyond that, split it into several batches.

What is the maximum pour depth?

Resin Glass 021+ pours up to about 20 centimetres. The 011 grades suit thinner work. For anything deeper, pour in layers with at least 2 hours between them.

How long does cobalt-promoted resin keep?

Roughly 1–4 weeks depending on storage temperature. If you are not using it soon, store it without cobalt.

Calculated? Time to Order

Use the calculator above to get your full material list, then tap the copy button and send it to us on LINE. The ResinSJ team will check the figures and recommend the right product for your job.

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