Maybe this is your first time being in this position – you finished letting a project cure and you went to work on it – and either it was cracked on arrival or as soon as it was handled it became this way. Although this is not an ideal incident, it doesn’t mean you have to throw your project out right away.
So, what exactly causes epoxy to crack? The most common reason is the epoxy got too hot while curing, causing it to cure faster and unevenly. The changes and differences in tempurature throughout the pour caused expanding and shrinking, which in turn cracked the already cured areas.
Why did it get too hot?
When the base resin is mixed with the curing agent, it causes an exothermic reaction and heats up greatly. While some epoxies are made to handle this, most will heat up exponentially when poured thick. Below I will discuss how (or if) you can fix it, and ways of improving future pours.
When to Not Fix the Cracked Epoxy?
If you ended up with cracked epoxy after a cure, don’t fear too much, there are often fixes for this. First, Here are a couple of possible reasons you would not fix it:
- If the epoxy did not cure fully, and is still tacky or sticky throughout the pour. This means the ratio was off, or the cure got way too hot and was unable to have the appropriate reactions.
- If the cracks cause the epoxy to “shatter” essentially making it a weak piece, you would not want to use this for risk of injury and damage.
How to Fix the Cracked Epoxy
If the cracks are on the surface (top or bottom), or if there are few cracks inside of the epoxy, or they are only along the edge, it is possible to salvage the piece.
For surface-level cracks:
You should be able to sand and fill as you would any other imperfections. This is also the way to add a second coat of epoxy.
1. Sand it down. Make sure when sanding you don’t sand too much – 320 would be plenty. Sanding it down too high of a grain can cause it to not have a good bond with the new epoxy being poured.
2. Use a air gun or a wet, clean rag to wipe off all extra residue on the surface. Make sure it is fully dry before continuing.
3. Pour the new epoxy over the entire surface, this will help give it a complete look again.
For edge cracks:
If they are surface level, I highly suggest the above steps. If they are inside the epoxy and abundant, there is a option. Personally, if it wasn’t an exact measurement needed for this piece, I would cut around the edge to just get rid of them, then sand and re-pour a top coat making the piece look new.
For cracks inside the piece:
If the cracks do not offer structural complications, you can leave them. They may even add an element of uniqueness. If it is a large crack, and (or) causing structural issues, you may be able to salvage it still.
1. If the crack stretches to the edge, take a saw and cut along the crack (or on either side and remove the piece cut out). If it is in the center of the piece, cut into it with a saw, router, chisel, or dremel and “hollow out” the area of the crack.
2. once it is a hollow area where the crack is/was, you are able to sand and pour just like the above.
3. After pouring the fill, do a top coat as well to help with the continuity of the piece.
How to Prevent Epoxy Cracking Next Pour
Thinner pours, more layers:
Likely, you were using an epoxy resin that was intended for less pour depth than you went for. It can be tempting, especially when not wanting to do as many pours, to try to stretch the limit of the pour. If the epoxy is rated at 3/8″ and you go for 1/2″ – you’re probably gonna be fine. If it’s rated for 1/4″ and you shoot for 1-2″, you’re likely to end up with cracked epoxy.
Lower the shop temp:
Although it’s tougher to control this, if you are doing larger pours, be careful not to let the shop get too hot. Cooler temperatures will cause a longer cure time, but will help mitigate the issue of a overheating layer.
Pour over metal:
Something that large-scale operations often do when pouring large tables (or always), is to pour over an aluminum or steel mold. The dense metal will help absorb the heat, unlike something wooden or plastic that would not do it as well. This is essentially how those meat-defroster trays work.
Use an epoxy made for deep pours:
With deep pours becoming more and more popular, many companies have come out with epoxies that are made to pour deep, sometimes up to 2.5″ even.