Previous preliminary testing within the European R&D project PROLIFE, (RFCS 2015-00025) of bolted lap joints loaded in shear has confirmed that small hard indenters inserted into a joint may indeed increase the effective friction of the joint. In order to further explore this feature, additional tests have been carried out with plates of different surface treatment and indenter size. Plates of structural steel grade S355+N were used in the experiments. Some plates were treated with fine blasting and others with a rougher treatment. Most plates were treated in the same way as steel components in new steel bridges in Sweden. For tests with the AMA surface treatment, 2.5 and 5.0 mm diameter stainless steel spherical indenters and 3.0 mm diameter silicon nitride spherical indenters were used. The necessary pre-loads to fully impress a 2.5 or a 5.0 mm stainless steel indenter into a steel plate surface are 10.6 and 41.0 kN, respectively, and for a 3.0 mm silicon nitride indenter 17.7 kN. In the lap joint tests an M24 bolt with a pre-loading force of 240 kN were used and the number of indenters adopted accordingly. The results were compared with each other and to slip factors according to Eurocode. The results confirm certain Eurocode friction coefficients and they further indicate which parameters need further research, in order to develop more efficient bolted friction lap joints.