One of the known drawbacks of in vitro dermal absorption methods is their high interlaboratory variation. Although often attributed to biological skin differences, it has been shown that validation of other parameters such as temperature and stirring speed can reduce the high variability observed. The Organisation for Economic Co-operation and Development (OECD) and, at the EU level, the Scientific Committee on Consumer Safety (SCCS) have published guidance documents of how to perform these in vitro tests. For the parameter 'sample application' and 'adequate seal', it is indicated to apply the sample homogeneously and provide an adequate seal between the donor chamber and the membrane on which the sample is applied. Here, a simple and visual densitometer-based method is provided, which makes evaluation possible of any application protocol used.