What is Dolly Test? Paint Testing
Dolly Test or paint test or coating test is carried out to evaluate the bonding strength with base metal or in other words to achieve the highest possible pull-off values from a pull of adhesion test.
Dolly Test is carried out by adhering or by glueing the test dollies on a metal surface that has been coated and left for curing as per the paint manufacturers recommendations or as per the product data sheet (PDS).
What is the Purpose of Doing a Pull-out Adhesion Test?
The purpose is to test the coating failure of a given test panel. It is a qualitative test and not a destructive test.
Dolly Test Acceptance Criteria
Dolly test acceptance criteria depend on the paint manufacturer and depending on the coating system and also it depends on whether it is for internal or external coating.
The acceptance and rejections criteria are usually specified in the project specifications.
It’s an expensive test that gives results in units of Psi, Newtons/m square.
Points To Note – Dolly Test
Scoring around the dolly is optional in ASTM D 4541 but recommended in ISO 4624, hence refer to the standards for confirmation.
How Do You Test Adhesion?
The test needs to be carried out as per the paint manufacturers recommendations and in accordance with the standards.
Typical Procedure – Dolly Test
Clean the test area and ensure that the surface is free from oil, grease, or any other contamination.
Gently abrade the dollies on the sandpaper.
The dollies can be of mild steel or of aluminium alloy that is also reusable.
Gently abrade the testing area with sandpaper
Mix the adhesives as per the manufacturer’s instructions.
Apply the glue uniformly on the entire dolly face.
Press the dollies onto the prepared surface applying even pressure on the dolly.
Remove the excess glue around the dolly.
Leave the dolly until cured using a dolly magnet or adhesive tape as per the manufacturer’s recommendations.
Adhesive cure time depends on the glue and according to the ambient temperature in the area.
Types of Failures – Assessing the Result of a Pull-Off Adhesion Test
1 Adhesive to dolly failure
2 Adhesive to aluminium failure
3 Cohesive failures within the aluminium
4 Aluminium to substrate failure
Testing Equipment – And Recording Results
Hydraulic Adhesion Testing Equipment (HATE) is the quickest and most accurate.
The dollies must be completely pulled off the surface and the readings must be recorded.
Assess the dolly face to determine the failure.
For a valid pull test, the dolly must be covered half of the area of the dolly face.
In case the glue fails and no coating is present on the dolly or covers less than the dolly face area the pull test is invalid and must be repeated.
If in case the value is higher than the desired project specification then it is valid.
Moreover, If the coating is failed within the layer leaving the same coating on the dolly and the test panel then it is a cohesive failure.
When either the coating is failed with the interference with other coatings (Between layers) leaving a coating on the dolly and other coatings on the substrate is known as adhesive.
Or when the coating is failed on the substrate leaving a coating on the dolly and bare substrate it is both cohesive and adhesive failure.